Add a removal method

[rfink]

Would love to have a method to remove either a given model or remove by index. I would happy to submit a pull request, if you would like.

[ianstormtaylor]

@calvinfo's got a PR coming with this one.

@visionmedia one question: how do you feel about renaming push to add for consistency with remove? we can add an alias if you want, for backwards compat

[tj]

sounds fine to me!