provides tenant-aware deface from a forked deface

A few notes:

* This commit uses a forked deface, so that should be thoroughly reviewed as well.
* I initially attempted to simply override the necessary deface methods, which mostly worked, until I got hopelessly shut down when trying to override the DSL::Loader (as the load method was already called via polyglot register before my override was executed).  I'm happy to go back to the override route if there's a clean way to do it.  Another challenge was 'overriding' the deface:precompile rake task (the existing task still gets executed, which is harmless, even if you redefine it in spree_landlord
* There's some hairy, questionable stuff going on in the spree_landlord Rakefile.  You'll need to run rake test_app, and you'll see some copying of files/assets.
* The specs all work, when run individually.  I got the point where i'm drained and want a 2nd set of eyes on the specs in order to help remedy.

Of particular interest, spec-wise, are the lib/tasks, deface, and controllers directories within spec