Infrastructure now in place to support cluster tests [IMMUTANT-261]

Additionally, we've collapsed a few options into a single 'modes'
property, which is a comma-delimited list of fntest modes. The default
is 'offset'. To run cluster tests, pass '-Dmodes=domain,offset'. That
will run every test beneath src/test/clojure/immutant/cluster. When not
in domain mode, all tests beneath src/test/clojure/immutant/integs will
be run, as before.

The other option worth mentioning is debug. Where before you passed
-Dfntest.debug=true, you will now include debug in your modes property,
e.g. -Dmodes=debug,offset,domain