options hash for glue function added

Hi, while using rabl I wondered why glue does not accept an if-lambda condition the same way as child and node do. I've seen that it was not implemented, yet, so here's my implementation. I also added 1 test case, which passes of course.

Hope you like it.

Thorsten

P.S.: This is my very first pull request - please don't ignore it if something's wrong. Instead, tell me about it!