Rename examples files, as per #288

This renames example files from `exampleN` to `example-description`.

Specifically, the following renaming is applied:

example1 -> example-methods-and-attributes
example2 -> example-python-types
example3 -> example-operator-overloading
example4 -> example-constants-and-functions
example5 -> example-callbacks (*)
example6 -> example-sequence-and-iterators
example7 -> example-buffers
example8 -> example-custom-ref-counting
example9 -> example-modules
example10 -> example-numpy-vectorize
example11 -> example-arg-keywords-and-defaults
example12 -> example-virtual-functions
example13 -> example-keep-alive
example14 -> example-opaque-types
example15 -> example-pickling
example16 -> example-inheritance
example17 -> example-stl-binders
example18 -> example-eval
example19 -> example-custom-exceptions

* the inheritance parts of example5 are moved into example-inheritance
(previously example16), and the remainder is left as example-callbacks.

This commit also renames the internal variables ("Example1",
"Example2", "Example4", etc.) into non-numeric names ("ExampleMandA",
"ExamplePythonTypes", "ExampleWithEnum", etc.) to correspond to the
file renaming.

The order of tests is preserved, but this can easily be changed if
there is some more natural ordering by updating the list in
examples/CMakeLists.txt.

example/example1.ref

-Called Example1 default constructor..
-Called Example1 constructor with value 32..
-Called Example1 copy constructor with value 32..
-Called Example1 copy constructor with value 32..
-Called Example1 destructor (32)
-Called Example1 destructor (32)
-Instance 1: Example1[value=320]
-Instance 2: Example1[value=32]
-Called Example1 copy constructor with value 320..
-Called Example1 move constructor with value 320..
-Called Example1 destructor (0)
-Example1[value=320]
-Called Example1 destructor (320)
-Example1[value=320]
-Example1[value=320]
-Example1[value=320]
-Example1[value=320]
-320
-320
-320
-320
-320
-Instance 1, direct access = 320
-Instance 1: Example1[value=100]
-Called Example1 destructor (32)
-Called Example1 destructor (100)

example/example12.ref

-Constructing Example12..
-Original implementation of Example12::run(state=10, value=20)
-30
-Caught expected exception: Tried to call pure virtual function "Example12::pure_virtual"
-Constructing Example12..
-ExtendedExample12::run(20), calling parent..
-Original implementation of Example12::run(state=11, value=21)
-32
-ExtendedExample12::run_bool()
-False
-ExtendedExample12::pure_virtual(): Hello world
-Destructing Example12..
-Destructing Example12..

example/example18.py

-from example import example18
-
-example18()
-
-

example/run_test.py

@@ -24,7 +24,7 @@ def sanitize(lines):
         line = line.replace('__builtin__', 'builtins')
         line = line.replace('example.', '')
         line = line.replace('unicode', 'str')
-        line = line.replace('Example4.EMode', 'EMode')
+        line = line.replace('ExampleWithEnum.EMode', 'EMode')
         line = line.replace('example.EMode', 'EMode')
         line = line.replace('method of builtins.PyCapsule instance', '')
         line = line.strip()