Fix a broken link to the MIT license
Fix symlink location in the installation guide
Update references to Propel 1.x generated folders

The folders generated with the propel-gen script aren't the same in
Propel 2. These folders are:

  * build/classes -> generated-classes
  * build/sql     -> generated-sql
  * build/conf    -> generated-conf

Update also the sub-structure of the generated-classes one.
Update the references to the 1.x repository

Update the documentation replacing all references to the "Propel" github
repository to match the actual one.