Added Servlet and clean-up/improved core files

Added:
GetRecentPostsServlet.java - retrieves most recent messages from chat server for a given room

Modfiied:
RequestParameters&ResponseParameters.java - defined new constants for added servlet and redefined existing ones to be clearer.
UserManager.java - removed unused imports
ChatManager.java - used appropriate timestamps
*servlets.java - modified servlets to use new resp and req parameters

web.xml - modified to accomodate new servlet

SubmittedBy: colinkho