GitLab

* expose save model to string to c_api

* add 'save-model-to-string' to python; use it to copy model

* remove boosting_type

1. add csc support
2. some data type from float to double

Categorical feature support (#108)

1. merge python-package
2. add dump model to json
3. fix bugs
4. clean code with pylint
5. update python examples

change static std::string to static char[]. (refer to https://google.github.io/styleguide/cppguide.html#Static_and_Global_Variables)