Fixes issue #21.  Adds the Serializable interface, updated the doc to include the new interface, and updated the documentation for IsSerializable.

Patch by: mmendez
Review by: bruce (desk check)



git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@922 8db76d5a-ed1c-0410-87a9-c151d255dfc7