Autosave and function to open sessions inside vim

Hey Raul!

Here is the promised pull request. Let me know if there's something you don't like. The s:session_loading var is needed because everytime vim leaves a buffer, the session is saved. And since when you load a new session, a lot of buffers are closed, several session saves will be triggered... Setting this var avoids that problem. But I'm not a vimscript expert, it may be better ways to do this :)

Cheers!