README: updates about omcache and fixes for examples
README: updates about omcache and fixes for examples

* Mention OMcache as an optional replacement for libmemcached
* Fix trigger example to not use `NEW` in a delete trigger
* Reformat API listing and clarify some comments about it
* Update build instructions
* Update Credits section

saaros/pgmemcache,ohmu/pgmemcache,ohmu/pgmemcache,ohmu/pgmemcache