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Adding documentation for value_error. Fixing various formatting issues. Removing redundant binding for C++ style methods. Adding bindings for iterator and slicing protocol. Extending examples.

This somewhat heavyweight solution will avoid size_t/long long/long/int
mismatches on various platforms once and for all. The previous template
overloads could e.g. not handle size_t on Darwin.

One gotcha: the 'format_descriptor<T>::value()' syntax changed to just
'format_descriptor<T>::value'