Example cpp file now has an example error

To demonstrate the diagnostic information response.
Diagnostic kinds now full name instead of 'E', 'W'

This changes both the C++ API and the REST API.
Returning full names for completion kinds
Allow usage of system Boost libraries
Allow usage of system GTest/GMock libraries
Merge pull request #7 from maksbotan/attempt2

System libraries, second try
Merge remote-tracking branch 'origin/master'