Extract flow feature into dedicated library

[daFish]

The flow feature should be extracted into a dedicated library to be able to use it in Non-Symfony2 projects. This will be a huge step forward in terms of reusability.
The bundle should only require the library and configure the services. Everything else belongs to the library.

There are 2 important points which need to be addressed in order to make this work:

• Commit history must be preserved
• Tests must pass

I propose the following approach:

• Create a new repository under CraueFormFlow
• Push the current code into the new repository (to maintain the commit history)
• Open a PR to remove all bundle-specific code from the library
• Open a PR here to remove all library-specific code from the bundle and to add the library as a dependency

It must be noted that this is a heavy BC break and will be available soonest with Version 3.0 of the bundle.

As @craue is already aware of this issue I'll start working on both PRs after he has created the new repository.

[craue]

No PRs left, new repo is created and code is pushed. Should be good to go. Your turn. 😏 I hope it will work as planned.

[craue]

Closing in favor of #147.