Ignore IntelliJ Idea .idea directory.

Change-Id: Ief5af0aa2267665e763972f39bd27e7ca6894e7b
Reviewed-on: http://review.couchbase.org/28738
Tested-by: Michael Nitschinger <michael.nitschinger@couchbase.com>
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>