Include description field in JSON serialization.

It looked to me like a lot of the heavy lifting had already been done for this, I just needed to finish off the "enrichWithBeanProperty" method such that it actually set the description on the schema object.  Where it got a little tricky was in dealing with the fact that this method was on JsonSchema, but the description field already existed on the SimpleTypeSchema class.  Not wanting to do a downcast, I was torn as to whether to move the "enrich" method to SimpleTypeSchema (requiring some cascading parameter type changes) or move the description to JsonSchema.  I tried both, and both work, but I liked the approach included in this pull request better.  If you'd be more interested in the approach where description is moved to JsonSchema I'm happy to submit that patch instead.  Just let me know :).
Nice catch.  You're right, it's not guaranteed to be SimpleTypeSchema.