Add ability to link a callback to a webstub, test stubs class callback
ability. References #21.
Add in integration spec for callback functionality within api spec.
Update readme and document usage of `with_callback` stub method
Merge pull request #22 from jonmorehouse/master

Add ability to link a callback to a webstub, test stubs class callback ability. References #21.
Version bump
Closes #23. Immutable array error on ios8