Added support for dynamic properties

Patch for issue #63 

Added tracking/creating inovcations for dynamic properties
The contribution based on the Tobias Krantzer's blog post - https://tobias-kraentzer.de/2013/05/15/dynamic-properties-in-objective-c/ and code - https://github.com/anagromataf/DynamicProperties

The main intention was to easily use Core Data mock object in tests. Let me know if these changes are redundant. I've tried to follow code convection style as well I've tried to cover new functionality with unit tests.

I'm new to obj-c development so please review changes carefully. Thank you!