Manage dependencies with CocoaPods

Instead of having submodules and manually keeping track of dependencies for each Sharer, we should use CocoaPods. Each Sharer becomes a pod subspec with its own dependencies. 

Integrating ShareKit into a project becomes as simple as adding this to the Podfile:
```
pod 'ShareKit/Facebook'
pod 'ShareKit/Twitter'
...
```

The included Podspec is based on the current Spec in https://github.com/CocoaPods/Specs but the repository needs to be tagged with the current version (2.2.1?) and the tag added to the Podspec.