Feature/settings api

@markoheijnen : What do you think about merging all options into one as an array? It would reduce mysql lookup (although caching is working here) and makes easier to integrate via the settings api
SEE https://github.com/cfoellmann/wordpress-mu-domain-mapping/commit/2b4c77e11b1683207f87ca2ee40d7a132d7e4604 -- works but is buggy. just want to know what you think before doing any more work in this direction