move mongod script to mongodb

Overriding the default mongod, especially in {prefix}/Cellar/mongodb/{version}/bin is, in my opinion, 100% wrong. I'm happy to discuss and justify this but I think a lot of the reasons why this is wrong should be self-apparent.

I do however love the little script that's there to automatically load the config.

My proposal is that the script remain but be renamed to mongodb and put into {bin}/mongodb so that we're not overriding the default mongod behavior.

I have only tested this with the devel branch of mongodb but with my limited understanding of homebrew I think it will work just fine with 2.4.9 as well.