Model JGwtCreate/JReboundEntryPoint request/result types as strings.

The actual types aren't important, the rebind logic is all about matching up request/result type.  Removing the types here makes things simpler.

http://gwt-code-reviews.appspot.com/1427808/

Review by: cromwellian@google.com

git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@10093 8db76d5a-ed1c-0410-87a9-c151d255dfc7