[service] Abstract: add patch method

Currently only updating sortId is supported
[ctrl] Abstracts: add patch(id) method

Format is described in RFC 6902
Special case JsNumber only for now
[route] add PATCH for /api/abstract/:id

Route PATCH (RFC 5789) to AbstractService::patch(id)
[test] test new patch methods in ctrl + service
[ctrl+srv+test] case classes for abstract PATCH op
Merge pull request #210 from gicmo/patch_op

Add support for changing sortId of abstracts

LGTM