Added a docs section on Linux installation

This simply adds a short paragraph on what dependencies are needed on
Linux, as well as the command line to install them on Ubuntu (since
users of other distributions are more likely to know how to do it).
Added Debian mention, extra missing packages

One tends to forget than not everyone has python-dev installed :)
Added Pypy note and fixed libffi's "spelling"
Pypy is not a real word either apparently.
Changed .... lines to ~~~~ and s/Gnu\/Linux/Linux/
Merge pull request #606 from chrisglass/installation-ubuntu

Added a docs section on Linux installation