Add BigDecimal/etc support.

This is what was previously rolled back in r7359 with a couple of
fixes.

Public review: http://gwt-code-reviews.appspot.com/126811

Patch by: jat
Review by: rice


git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@7618 8db76d5a-ed1c-0410-87a9-c151d255dfc7