Add Podspec

I've created an initial podspec for this project which needs a few things before it's ready to publish:

* A version number tag, which should match the value of `s.version` (change this to 1.0 or whatever you want)
* The platform target version and other values should be reviewed

Once you have a tag, you can validate the spec using `pod spec lint`. The spec won't successfully lint until a tag exists on your repo that matches the version number.

Then, get yourself set up with [CocoaPods Trunk](http://guides.cocoapods.org/making/getting-setup-with-trunk.html) and publish this spec using the command `pod trunk push SmudgeKit.podspec`.

Addresses #2.