Add Shift+Tab map for tab complete in insert mode

Feature parity with `<c-p>` `<c-n>` tab completion duo.
Set Vim's colorcolumn to 80 characters

* We have an 80 character formatting limit
* This makes it obvious where that limit is
Adds `vim-ruby` back to `vimrc.bundles`

Removal in https://github.com/thoughtbot/dotfiles/commit/deb7ee5948a0df56c2a68fc2dab22fd31d70a204
was accidental.

Also, removes `vim/bundle` directory as it is no longer useful to us, since
`vundle` creates it when it needs to.
Allow stylesheets to autocomplete hyphenated words
Add /usr/local/bin to PATH before /usr/bin

This is for homebrew binaries to work.

Load `rbenv` after adding `$HOME/.bin` and `/usr/local/bin` to `PATH`, but
before adding project-specific binstubs.
tat command now works in directories with periods

- Running `tat` in the robots.thoughtbot.com directory was throwing a
'bad session name' tmux error
- See http://sourceforge.net/p/tmux/tickets/109/
Set Vim's spellfile to $HOME

Running `zg` adds words to the `spellfile`:

https://github.com/thoughtbot/dotfiles/commit/4f5a2edc336970b8a1c7eded3934182810abeb20
http://robots.thoughtbot.com/vim-spell-checking

Setting the spellfile keeps it out of its default location, `vim/spell`, which
would otherwise be inside thoughtbot/dotfiles. We don't necessarily want to
share the `spellfile` across the team.
Update for Vundle's new interface

* Vundle now lives at gmarik/Vundle.vim

See https://github.com/gmarik/Vundle.vim/blob/master/doc/vundle.txt#L345-L369
Make it easy to checkout GitHub pull requests

This alias makes the following workflow possible:

Look at changes:

    git co-pr 123
    git diff origin/master

Bundle, validate tests are passing:

    bundle
    rake

Rebase interactively, squash, and potentially improve commit messages:

    git rebase -i master

Merge code into master:

    git checkout master
    git merge pr/123 --ff-only

Push:

    git push origin master

Clean up:

    git branch -D pr/123
Allow personal overrides to psqlrc

I wanted to add my own customizations after reading @mike-burns' post on
[`psqlrc`](http://robots.thoughtbot.com/an-explained-psqlrc).

As a result of this change, `.psqlrc` requires that a `.psqlrc.local`
exist in the home folder because `psql` cannot do conditional sourcing
of files in the style that we use in other foundation dotfiles. Touching
the file to ensure it exists (even if blank) prevents `psql` from
exiting with an error if the user does not provide their own custom
configuration.

Additional changes:
* Updating the documentation with regard to which files get excluded.
Merge remote-tracking branch 'upstream/master'