Update Core Data & example test.

This update:

* adds the Pods library to the tests target.
* allows an environment variable to specify the database name (only in debug build configuration)
* adds a shared scheme with a test database environment set for when running tests.
* adds an example test using core data.

There is an update to the podfile means everyone will need to run `pod install` again.

Also, if you experience a crash in a Core Data call, it is likely that your .sqlite data file is corrupt. Simply deleting the file should fix it (or the whole app from your phone / simulator).