Move commands into the Contracts assembly

[gmelnik]

As per user's suggestion from http://cqrs.uservoice.com/forums/149875-general/suggestions/2824295-move-your-commands-into-the-contracts-assembly :

The client holds references to your service implementation projects, this blurs out what is actually the public surface of your services. Move the things that should be consumed by clients into the Contracts assemblies to make versioning simpler and dependencies more explicit."