Added optionally bundling YouCompleteMe.

So when I originally started writing this I was going to test for the binary like you suggested however I hit a bit of a catch 22. 

In order for the ycm_core to be available the plugin would have already needed to be bundled for the source to be available for compilation. As such it does not seem legit to check for the ycm_core before issuing the Bundle command or new users with fresh installs would never bundle it (without going out of their way to do it manually the first time to compile the core). 

I also didn't really like the idea of bundling it only to unbundle it straight after should the compiled core not be found. This approach could have left users confused as they might think they are using YouCompleteMe when it is in fact falling back to Neocomplcache or something similar.

At the moment my recommended approach would be to allow users to bundle it in their own .vimrc.bundles and then the onus is on them to then make sure the core is compiled. The plugin itself actually performs checks and tells the user if the core cannot be found. 

If you can think of a better way to do this I'm happy to make changes.

EDIT: I'm not really intending that you merge this straight away. Just wanted to start a discussion about it's implementation.