Add wheezy to the list of OSes

Extend the locale instructions to either version of Debian.
More descriptive heading and make headings same level
Note requirement for root privileges
Add wheezy repo for squeeze

Give better description of what we're getting from each repository and
suggest package pinning for wheezy.
Add section for wheezy.
Add raring repository for precise.

This allows us to get a more recent version of bundler and pdftk. Also
add package pinning instructions. Switch repo as EC2 was giving 403 on
some packages.
Clarify instructions for installing mysociety versions of packages

Add wheezy, don't overwrite /etc/apt/preferences if it's already there. Add section for squeeze that says you don't need to do anything.
Update instructions on manually installing tricky package dependencies
Add wheezy install instructions for the package file
We're backporting bundler for Ubuntu.

Update the instructions to reflect that.