Allow for inheritance in SugarRecord subclasses

I wanted to have the ability to create my data model using an abstract class that extends from SugarRecord, and have my actual types to store in the database extend from that abstract class.  In order to do this, I have updated SugarRecord and SugarDb to check if the discovered classes are "assignable from" the SugarRecord class instead of just checking to see that the superclass is SugarRecord.  I also updated SugarRecord to find all fields in the current class, and all the way up the class hierarchy.

Let me know if you have any comments on my implementation of this.