Implement DataDisplay and SymbolTableNotifier

Currently DataDisplay works only for newly inserted symbol.

SymbolTableNotifier is an observable and DataDisplay is a corresponding
observer. cf. Observer Pattern. This observeable needs to register
DataDisplay before it starts inserting symbols. See Driver (main()) in
DataDisplay for usage.
Merge branch 'master' of github.com:citrin2013/citrin

Conflicts:
	SymbolTableNotifier.java