Updated Readme

Readme now
- Has a new description attempting to answer why a developer would use Pivotal Core Kit
- Highlights some of the more universally useful functionality
- Describes use cases for Core, Spec, and Stubs
- Provides a somewhat exhaustive list of functionality (I didn't know PCK could do most of these things before making this list)
- Provides a few code examples of using PCK in a project.
- Is hopefully a little more clear on installation (I'm not including cocoapods directions in this pull request)
- Updates copyright year and contact info