Extract coordinator.Permissions

All ClusterAdmins now qualify as DbAdmins too.

I think the discussion in Issue #313 is indicative of a smell that the coordinator object shouldn't directly contain the logic about deciding whether a user has permissions to perform a particular operation -- hence extracting a permissions object that is lightweight for now, but should be flexible enough to have finer-grained permissions in the future if desired.

I added a TODO to a couple of the tests that I added, where it wasn't extremely obvious to me what the extra logic was doing. It will probably be trivial to address for anyone who's more familiar with user roles and permissions of InfluxDB.

Also, I'm having trouble running the `./configure --with-flex=/usr/local/Cellar/flex/2.5.37/bin/flex --with-bison=/usr/local/Cellar/bison/3.0.2/bin/bison && make` command as recommended in the dev guide (I'm on OSX 10.9.2). It's possible I may have broken some tests outside the coordinator package, although I don't expect that to be the case.