Added polysquare_source_type_from_source_file_extension.

This function does shallow analysis to determine the type of a source
file (valid types: C_SOURCE CXX_SOURCE HEADER UNKNOWN) based on its
extension.
Merge pull request #9 from smspillaz/cmake-determine-header-language.item_9

Added polysquare_source_type_from_source_file_extension.