Merge pull request #74 from stephenjust/model-code

More Question & Answer model code
Merge pull request #75 from stephenjust/localdatastore

Localdatastore
Add Gson library
Make model objects serializable
Add toString methods to Question, Comment, Answer classes.
Add equals() to Question, Answer, Comment classes.
Add data serialization and extra consistency checks to LocalDataStore.
Fix tests, and test more LocalDataStore.