[WIP] Propel2 integration

I started working on integrating Propel2 in this bundle. Even if the work is at its early stage, there may be people interested in testing it or even in contributing.

Here is the ToDo list:

 * [x] ParamConverter
 * [x] Web debug toolbar
 * [ ] ACL Implementation
 * Commands:
   * [x] propel:build
   * [x] propel:database:reverse
   * [x] propel:graphviz:generate
   * [x] propel:migration:down
   * [x] propel:migration:status
   * [x] propel:migration:up
   * [x] propel:model:build
   * [x] propel:sql:build
   * [x] propel:sql:insert
   * [ ] propel:acl:init
   * [ ] propel:database:drop
   * [ ] propel:database:create
   * [ ] propel:fixtures:dump
   * [ ] propel:fixtures:load
   * [ ] propel:table:drop
   * [ ] propel:form:generate
 * Fixtures-related:
   * [ ] dumping
   * [ ] loading
 * Form-related (PropelExtension):
   * [x] TypeGuesser
   * [ ] ModelChoiceList
   * [ ] CollectionToArrayTransformer
   * [ ] Translation-related event listeners
   * [ ] CollectionToArrayTransformer
   * [ ] ModelType
   * [ ] TranslationCollectionType
   * [ ] TranslationType
 * [ ] and last but not least, tests! Right now and even if the ParamConverter seems to work properly in my test application, its tests fail.

As you can see, there is still some work to be done and of course, I'd be happy to accept any help/code review/feedback/advice :)

P.S : some files are copied from Propel's bridge in Symfony. I just made a few fixes in them and I left their licence header, is that the right way to it?