Support for multiple template repositories

A working first stab at support for multiple template repositories. Main aim is to allow for internal company repositories so that company specific templates needn't pollute the public repository.
here I have a UI question about how multiple template catalogs would show up in Activator - we may want to get @Warry 's input - if we want to show the multiple catalogs as such to the user, then we might need to maintain them as separate entities above this level, while if we want to merge them all and make them look like a single catalog, we want to do that at the lowest layer possible (probably this actor is as good as any). A middle ground is to merge them but extend the metadata to contain the originating repository information.

Another UI question: do we need a little more info on the repositories, such as a logo / description, or a link to the online catalog when one is available such as the one on typesafe.com ...