Implemented Unregister and ChangePassword functionality

Implemented Servlets that allow Unregistering and ChangePassword.

Added RequestParameters.java
* Central Utility class that holds String constants that denote requests from clients

Added PasswordServlet.java
* Processes a change of password request for a given user.

Modified web.xml
* Modified to allow url to direct to newly implemented servlets

Modified RegisterServlet
* Modified RegisterServlet to use RequestParameter for cleaning code

ModifiedBy: colinkho