implementation of a doubly linked list using generic types
1.Renamed some of the files and put into packages for better organisation
2.Junit practice incorporated into this commit, including a test class for hash function, as well as test classes for integer stack and 2
different implementations of String stack(package String Stack)
3. Generic map and stack implemented are part of this commit
Partial implementation for a hash table, the method for data retrieval not committed yet, will be submitted with next commit