references inside field/enumvalue options

git-svn-id: http://protostuff.googlecode.com/svn/trunk@1616 3b38d51a-9144-11de-8ad7-8b2c2f1e2016
reuse

git-svn-id: http://protostuff.googlecode.com/svn/trunk@1617 3b38d51a-9144-11de-8ad7-8b2c2f1e2016
empty check

git-svn-id: http://protostuff.googlecode.com/svn/trunk@1618 3b38d51a-9144-11de-8ad7-8b2c2f1e2016