Ignore intellij directory on ant clean.

Change-Id: If564fdb8eee84ff2f1fe39718588bbe5fb98f4a6
Reviewed-on: http://review.couchbase.org/28765
Tested-by: Michael Nitschinger <michael.nitschinger@couchbase.com>
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>