Fix for incorrect exception message when getting context by name and fix for delegating getting Id to parent if null locally

git-svn-id: https://svn.forgerock.org/openig/branches/http-framework@720 dbb9e58e-28e6-4ce0-90e8-f11d9605b710
Moving Context classes from http-core to resource-core so they can be used from json-resource

git-svn-id: https://svn.forgerock.org/openig/branches/http-framework@721 dbb9e58e-28e6-4ce0-90e8-f11d9605b710