wagn installation and testing

(not yet finished)

The main goal of this PR is to be able to test mods in decks. 
In order to achieve this we try to get a cleaner way to install wagn for mod developers and core developers.

The procedure at the moment with this PR is:

       wagn new devdeck --core-dev 

sets up a new deck "devdeck"  configured such that in devdeck the specs and cukes for the wagn core can be run (via `wagn rspec` and `wagn cucumber`). This has the advantage that we need no longer the possibility to install a deck inside of the wagn gem for core testing and we can get rid of a lot of the rails configuration files. The default configuration for decks is now stored in lib/wagn/config and can be overwritten or adapted in the deck's config directory.

PR introduces also a `wagn seed` command  to complete the installation process (instead of `rake wagn:create`)