Replaced Syntastic support with YCM-native code

Currently, the only supported Syntastic features are the error signs in the
gutter. Other features will be added in the future.
Echoing diag text on when cursor on diag line
FAQ item about dropped Syntastic support
Now showing red squiggle under diagnostic location
Diagnostic struct now holds source ranges

Also, line/column/file info is now in Location. The server API is the same
currently, but will change to reflect all of this soon.