Make sure ant clean does not wipe .idea directory files.

Change-Id: I0db0906f36a2018e8ed918fa8e99aa2ec9cc7f01
Reviewed-on: http://review.couchbase.org/28764
Tested-by: Michael Nitschinger <michael.nitschinger@couchbase.com>
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>