Content types added into header while serving static files

[cong1920]

• Added a step in CMake to generate file content_types.hh to have a full list of common static files' content type strings. The list is stored as unordered_map<> for quick search in code. The sources file is actually http://svn.apache.org/repos/asf/httpd/httpd/trunk/docs/conf/mime.types
• In http_serve.hh: send_static_file(const char* path), added some code to parse out file extension and use it to search its content type string, store it along with file content in unordered_map<> static_files, and output in header before sending content.

[matt-42]

Thanks @cong1920 for the PR. Looks good, I'm reviewing it.