Omnifunc & Autoactivation improvements

Hi, I had done some very small but very useful improvements that I think are worth sharing.
1) Make completion use omnifunc instead of basic completion. That gives you, for example, integration wit YouCompleteMe wight out of the box.
2) Plugin autoactivation. Instead of relying on activating plugin's functionality for some filetypes all the time, just enable it for all files but only when inside an idea project. This is, for any file who has an ancestor directory containing an .idea/ directory.