Add podspec for URLMock 1.1

URLMock 1.1 is a minor update that adds the following functionality:

* Better unexpected request detection when verification is enabled
* Better request/response description methods to aid in debugging failures
* New convenience methods for adding mock HTTP requests and responses with JSON bodies
* New test utility functions for generating arrays, dictionaries, and sets with blocks. These are great for randomly generating unit test data. These and other test utilities are available in the TestHelpers Pod subspec.
* New SubclassResponsibilty Pod subspec for easily throwing NSExceptions when a subclass fails to override an abstract method.