Preprocessor defines for serialization function names

The following preprocessor defines have been added:

 - CEREAL_SERIALIZE_FUNCTION_NAME
 - CEREAL_SAVE_FUNCTION_NAME
 - CEREAL_LOAD_FUNCTION_NAME

These defines specifiy the name of the cereal serialization/deserialization
functions so that they can be customised by users. This is especially useful if
the user would like to have capitialized function names.