adding md5file action
added md5.cpp and md5.h to projects
Added std::string get_file_extension(std::string&) and int compare_file_modified_time(std::string, std::string) to utility functions.

Added action_md5file(asset*, std::string, std::string) for writing individual md5 files for matching files.
added time_t file_modified_time(std::string).
completed implementation of action_md5file action.
Merge branch 'md5file'