Dev/windowingops

[mstaylor]

Niranda,

This is an initial PR that includes rolling windowing operations for review and feedback. I've cross-referenced the pandas windowing apis with the intention of making our implementation consistent with what is there. This PR includes the following:

1. Support for CYLON_DEBUG via cmake switch
2. Windowing operation namespace introduction and CMakefile
3. hash_window implementation for fixed number of operations (slice and dice suggested implementation)
4. window_example and CMakefile
5. Other required CMakefile modifications