Fix post to superMasterBackend

We need to separate the individual rrs in the rrset.
Added isMaster declaration
isMaster implementation
Support for isMaster
Tests for isMaster
Documentation for isMaster
Change isMaster to GET
Send Accept: application/json header on request
Do not prepare isMaster as post
Merge pull request #1542 from rubenk/fix-supermaster-backend-post-query

Fix POST to /superMasterBackend
Merge pull request #1544 from cmouse/remotebackend-ismaster-support

Remotebackend ismaster support
Merge pull request #1545 from cmouse/remotebackend-acccept-header

Send Accept: application/json header on request
Fix exception handling and use "" for NULL
Few more sanity checks to avoid aborts