Marking specific dates

Hi. I've added the ability to mark specific dates:

![Screen Shot 2013-04-06 at 11 48 39](https://f.cloud.github.com/assets/625199/347197/4b3cb484-9ea8-11e2-9996-65bb1f716399.png)

It's not possible to mark dates outside the minimum and maximum range, and the selected date overrides marking. In the above image, the 19th and 21st are marked.

I needed this feature for a project I'm working on, so I've added the bare essentials to get it working. It could probably be neater.

I've also added a podspec for using the forked repo with CocoaPods. Feel free to remove it when merging, or modify it for the main repo (I've included it in the pull request in the hopes you'll do the latter).