Remove all {% highlight %} tags from the documentation

Use instead Markdown-like syntax for code blocks. Add a plugin to allow
this and a Gemfile in order to install the required dependencies.
Rewrite the installation guide

Show how to setup Propel via several ways such as with Git or Composer.
Also fix some typo and remove deprecated stuff (such as sentences
mentionning Phing).
Some changes in the config stuff
Rewrite the "Buildtime" guide

Update the guide for Propel2 changing command names and few other fixes
Little fix in the third guide