Rename SaveSession, OpenSession, etc. to SessionSave, SessionOpen, etc.

[mawkler]

I personally think it would make more sense if the session commands had the prefix "Session", instead of having them as a suffix. I just started using this plugin and the first thing I did when trying it out was to type :Session and then press <Tab> to see the available commands, which of course gave no results. I think the plugin would be easier to learn if all the session related commands showed up when tab-completing :Session, since now I have to constantly do :h session-commands to see the available commands.

This is kind of a big change user-interface-wise, so I totally understand if you don't want to change this.