Update Rakefile

* Upgraded PE from 3.0.1 to 3.1.0
* Upgraded CentOS from 6.3 to 6.4
Making the iso_url for CentOS more generic.
Fix the Debian iso_url and actually fix the CentOS iso_url.
Setting the background in vim to dark by default.
Merge pull request #161 from klynton/improvement/vim-bg-fix

Setting the background in vim to dark by default.
Merge pull request #154 from klynton/bugfix-centosupgrade

Update PE and CentOS iso_url in Rakefile.
Added virtualbox, xorriso to initial setup, specified 32-bit CentOS ISOs
Punctuation cleanup
Merge pull request #162 from gsarjeant/readme_add_setup_steps

Add steps to the "Initial Setup" section of the README

Thanks @gsarjeant.
Adding a note about using homebrew.
Merge pull request #163 from klynton/enhancement/brew-note

Adding a note about using homebrew.
Thanks @klynton :+1:
the :: in the filename messes up Gepetto
Merge pull request #164 from casharma/fix/fund/tests

the :: in the filename messes up Gepetto