[#1029] Add simple API for managing custom fields.
[#1029] Add simple API for managing custom fields.

Also, allow for no custom fields defined.

heiths/allura,apache/allura,apache/incubator-allura,heiths/allura,apache/incubator-allura,apache/allura,Bitergia/allura,apache/incubator-allura,apache/allura,lym/allura-git,Bitergia/allura,lym/allura-git,heiths/allura,apache/allura,leotrubach/sourceforge-allura,Bitergia/allura,apache/incubator-allura,leotrubach/sourceforge-allura,Bitergia/allura,lym/allura-git,lym/allura-git,lym/allura-git,apache/allura,Bitergia/allura,heiths/allura,heiths/allura,leotrubach/sourceforge-allura,leotrubach/sourceforge-allura