Complete refactoring of the bundle

This version is a complete restructuring of the codebase, and brings along with it a few breaking changes.

The main changes are:

• the database table used to store migrations has changed. It uses different columns, and a different name by default, to avoid conflicts with the previous table

• the bundle should now work on PostgreSQL, or any other database supported by Doctrine - it has only been tested on MySQL though :-)

• naming change: what was previously called a version is now called a migration (in the docs, error messages, source code, etc...)

• the generate command takes an optional 2nd argument, it make it easier to create migration definition files with a meaningful name other than "placeholder".
  The options it takes also changed and behave differently from before.

• the status command displays much more information than before:

  • the date that migrations have been executed
  • the reason for migration failure
  • any differences between migrations definitions and the definitions used at the time the migrations were executed

  It also lists all migrations: both the ones available on disk as definition files, and the ones stored in the database (previously if you deleted a migration definition, it would just not be listed any more)

• the update command has been renamed to migrate (but the previous name will be kept as alias for a while)

• the migrate command now prevents concurrent execution of the same migration, stores in the database the reason of failure of execution, warns of invalid migration definitions before execution, makes proper usage of database transactions and probably more

• the location/create action now allows a more flexible way to identify the objects to which the new location is to be added. This will be extended to all other actions in the future

• php migrations are now fully supported (they used to have naming problems)

• the validity of migration definition files is now checked before migration execution is attempted

• the console commands now give more detailed, helpful error messages

• it is much easier now to extend the bundle, as proper Dependency Injection is used everywhere, as well as tagged services