Flat Token Design Support & Podspec file

In this PR I've added support for flat color tokens (i.e no gradients). When the token is selected/highlighted the color is adjusted to be either slightly lighter or darker dependent on a weighted brightness of the token's tintcolor. See the image below for an example. 

For those who don't know, you can adjust your token's tintColor in the following delegate:
- (BOOL)tokenField:(TITokenField *)tokenField willAddToken:(TIToken *)token

I also added the podspec file as this is fairly standard across pods to have it in the repo. (I would say at least 90% of the pods I've used / looked at have it this way)

![flat_tokens](https://cloud.githubusercontent.com/assets/969761/3534709/bb7b359c-07f0-11e4-8fdf-f93cbb46cb97.png)