Additions from JS Callbacks Document

All changes done except SWF Parameters Hostname TestSuite
Changes from comments set#1
Changes from comments set#2

1. Removed unnecessary import
2. Added HTML tag constants to NDTConstants
3. Restructured function initializeFromHTML
Merge pull request #4 from anantsubramanian/js-callbacks

Js callbacks
Minor comment change, to make it clear that message types cannot be changed without changing the NDT server.
Renamed and re-organized some constants for clarity.
Minor style changes.
Minor style changes.
Defined implicit getter for message type, to avoid inconsistancy with the value of the first byte of the body.
Defined getter for message body.
Remove unused code.
Fixed mistake related to message type setter.
Moved recv_msg to Message.as. Added class var length.
Added constants for return values of receiveMessage.
Moved readn to Message.
Added to Message support for msg KICK_OLD_CLIENTS
Disable context manu even if the GUI is not enabled.
Moved readBytes to NDTUtils.
Initialize Message class variables.
Removed initBodySize.