Fixed the dll import for relative paths + various cleanup.
Fixed the dll import for relative paths + various cleanup.

- DLL import now works when __file__ is a relative path
- Various PEP8 and whitespace fixes + whitespace cleanup
- Docstring fixes (conform to numpydoc)
- Added __all__ to the module
- Fixed mutable default values
- Removed print statements
- py2/py3-compatible string-type checks
- Replace asserts with proper exceptions
- Make classes new-style (derive from object)

dmlc/xgboost,dmlc/xgboost,dmlc/xgboost,dmlc/xgboost,dmlc/xgboost,dmlc/xgboost