YouCompleteMe support for Syntastic

YouCompleteMe is an about-to-be-released (within days) open-source code-completion plugin for Vim (https://github.com/Valloric/YouCompleteMe). It has semantic completion support for C/C++/ObjC/ObjC++ (similar to clang_complete, but much better).

YCM compiles the current source file in a background thread whenever it is changed (it does not need to be saved). This is necessary to offer semantic completion candidate strings. Clang is used for the compilation through the libclang bindings. 

Since a parsed version of the file is always available, YCM offers a way to hand any detected errors/warnings to Syntastic for display. By using both YCM and Syntastic at the same time, the user has already opted into the background compilation so piping the issues to Syntastic is a good idea.