Initial conversion to dynamic library.

All keywords are still contained within a single class, but this will change as
I add more keywords and organize existing ones.
Minor formatting changes, combined tests
Experimental changes to implement subscribe keyword

Also moved back the keywords to MQTTLibrary class to make it simpler for
now. If the class becomes too big to hold the keywords, then it can be
broken down into different classes.

A test has been added to test the subscribe and validate functionality.