1963 eunit bigcouch

This adds a rebar plugin to bootstrap to tmp test environment with data folders and the relevant ini files. To run this right now you'll need to `cp src/couch/priv/couchjs bin/couchjs` until we figure out how to handle that properly.

One issue with the setup_eunit command is that the rebar templates require all variables to be defined in order to overwrite them. So I've set default destinations of `/tmp`, which I'm not convinced is great. The test suite won't run, but it will create the files and folders in there. One hacky solution would be to use `/somelongdirectorythatdoesntexist` that will cause rebar to explode, but I'm not a huge fan of that approach.
Thanks @hdiedrich, fixed in 9024342.