Add support for OPTIONS method

This method simply sends the support methods, content types, etc.
Currently all methods that are supported in server.cpp (not commented out)
are the support methods. Once we implement other functionality,
options.cpp will need to be updated.

Tested using curl and works as expected.
This (among other files) is included in both options.h and options.cpp - it should only need to be included once.

As a general rule of thumb, only include things that are absolutely necessary in .h files - include everything else in the corresponding .cpp file.