Don't remember cursor position for commit messages

It's not very useful to be dropped into the middle of your diff when
opening a new commit. This change prevents cursor position from being
restored for the `gitcommit` filetype.
Add syntastic for syntax checking

* enable check on open: I want to know about any broken windows from the
  start.

* disable inline highlights: the line markers are sufficient.
Extract dev/staging/prod binaries to parity gem

https://github.com/croaky/parity
Add mappings to switch windows more succinctly
Remove reattach-to-user-namespace

This is Mac OS X-specific. Including this line in the dotfiles causes
`tmux` and `tmux new -s new-session` to break with `[exited]`.

I personally don't do copy-pasteable work inside tmux anymore.

I'm sure this is valuable to some of the thoughtbot vim users but I'm
thinking it better belongs above `DO NOT EDIT BELOW THIS LINE` as custom
configuration for those users.
Consolidate autocommands into the augroup

This makes it so resourcing the vimrc file doesn't duplicate the
auto commands.
Add ag and update for z