abc-bank updated from hc

1. Added new feature Transfer Account. For that refactoring class bank and customer to use LinkedHashMap.
So we can fast to search a customer or account.

2. Fix the string building using StringBuilder, instead of "+"

3. Made DateProvider and Account thread safe 

4. Added lots of new unit Testing.

5. Create New tesing class for Account.

6. Fixed null pointer exception from class Bank.