Modified IE driver server to more closely follow W3C spec

This commit includes a number of changes designed to bring the IE driver
into closer alignment with the W3C WebDriver specification. It provides
no functional changes to the driver, nor does it change any external-
facing API. The changes are:
 * Changed webdriver::Server to examine the response from the newSession
   command for the session ID rather than a two-stage process. The
   webdriver::Server::DispatchCommand method now calls the
   InitializeSession method directly when processing the newSession
   command.
 * Removed the now-obsolete webdriver::Server::CreateSession method.
 * Added a Serialize/Deserialize method pair on the webdriver::Command
   class (renaming the Populate method to Deserialize).
 * Revamped the serialization of webdriver::Command to use 'name' instead
   of 'command' for the command name.
 * Added a session ID member to the webdriver::Command object.
 * Modified the webdriver::Command object to no longer draw distinction
   between parameters passed in as part of the URL substitution and those
   passed in as part of the JSON payload in the body.
 * Modified webdriver::CommandHandler::ExecuteInternal (and all subclass
   implementations to use a single parameters map instead of the dual
   URL tokens/JSON payload parameters maps used previously.