Implement JsonPath in JsonReader.

git-svn-id: http://google-gson.googlecode.com/svn/trunk@1286 2534bb62-2c4b-0410-85e8-b5006b95c4ae
incorporated code review feedback. Added a test to ensure JsonAdapter validation doesn't carry side-effects to other fields

git-svn-id: http://google-gson.googlecode.com/svn/trunk@1287 2534bb62-2c4b-0410-85e8-b5006b95c4ae