Releasing v0.2

* Switchined to using VimEnter autocmd for loading autocmd to do it more
  gracefully, otherwise there were side effects like skipping of -c commands.
* Changed variable name `g:prosession_load_on_startup` to
  `g:prosession_on_startup`.
* Added a guard condition within s:Prosession to ensure we stop a session only
  if it's already started to avoid side effects.