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.
Fix homebrew conditional
Add rcm

The rcm suite of tools are used to manage dotfiles. They can symlink
dotfiles from a repo into your homedir, turn an existing configuration
file into a symlinked dotfile, and in general make it easy to try new
dotfile repos.
Use thoughtbot/rcm for Homebrew

Back when rcm was an experiment by Mike Burns, the `homebrew-rcm` GitHub
repo was hosted under the `mike-burns` account. It has since been moved
to `thoughtbot` so that more may share in the love.
Change Homebrew install path

Looks like it's hosted in a different repository now.
Add command for accepting XCode agreement on Mavericks
Upgrade Ruby to 2.1.0
Remove initdb, fixes #182

The latest Homebrew formula runs this as part of post-install.
Improve idempotency in the linux laptop script

These changes allow the laptop script to run more than once and can be
tested through ./test/runner.sh.  The only significant side effects are:

* Gems (pg, rails, unicorn) will be updated
* ruby $RUBY_VERSION will be set as the global ruby if it wasn't already
* rubygems proper will be updated to the latest release
* hub will be updated
* the heroku toolbelt will be updated.
Update license to 2014