Do not rehash on rbenv initialization

We install rbenv-gem-rehash, which will automatically rehash when you
install new binaries, so I can't conceive of why this is necessary. I've been
using --no-rehash for months now without issue, and there's a noticeable speed
up in shell initialization.
Add libreadline-dev to linux install

Without libreadline-dev installed, IRB and the rails console won't
function properly.
Update README.md about Ubuntu 13.10 support
Add C compiler installation instructions for 10.9

Reformat C compiler instructions for various OS X versions so people can scan by
their version of OS X.
Fix QT on Mavericks
Change brew url.
rejiggered homebrew for #156
Make Homebrew install idempotent
Use updated Qt formula for Homebrew

The `--HEAD` flag is no longer necessary:

https://github.com/mxcl/homebrew/commit/4bd60168ffbff7246656fcd737cb1f26407afd54
Use Ruby 2.0.0.p353 as default

Resolves reported vulnerability in p247.

https://www.ruby-lang.org/en/news/2013/11/22/heap-overflow-in-floating-p
oint-parsing-cve-2013-4164/
Configure Bundler for parallel gem installs

* Set it globally for OS X.
* Determine number of cores dynamically.
* Pick one number less than number of cores to avoid deadlock errors.
  http://archlever.blogspot.com/2013/09/lies-damned-lies-and-truths-backed-by.html
* Only install `--pre` for Bundler.
* Remove `pg` and `unicorn` gems as they will be installed during `bundle`
  for a Rails project.
Merge branch 'refs/heads/thoughtbot'