Set up Doxygen to work with the CMake tool.

Builds the Doxygen and stores in docs/html directory.

$ cmake . 
$ make doc

Fixed up all warnings in the Doxygen comments in cassandra.h file.
TBD: 3 typedef structs are showing up in the classes listing.
All of the other global C functions and typedefs are showing up at:
cpp-driver/docs/html/cassandra_8h.html