Update src/csci4370project3/BpTree.java

Added test cases for all three submaps 
Tentative tailMap and subMap (no testing)

Implemented tail map by essentially reversing headMap. (Not at all confident)
Implemented subMap by using headMap and tailMap. (Very confident)
No testing has been performed yet. 
Merge pull request #3 from jeffswindle/subMaps

Tail Map and Sub Map without Testing