MongoMapper Associations 1.0

Hello, I have updated the MongoMapper implementation from pull request [45](https://github.com/michaeldv/awesome_print/pull/45) to be compatible with the latest release `awesome_print` version `1.0.2`.

I've also made the tests from the original pull request to compatible with the new structure in the `1.0.2` version of `awesome_print`.

I've also made the gem compatible with bundler and able to be run on [travis-ci.org](http://travis-ci.org/#!/gardelea/awesome_print) and updated [README.md](https://github.com/gardelea/awesome_print/tree/mongo-mapper-associations-1.0).  i can break these out to a different pull request if they are an issue.

Mainly wanted to get the dialog started on how, and even if you would consider this pull request.

thanks for all the awesome work !