misc fixes
* move couchbase.config to raddb/mods-available/couchbase and remove
install section for this file from all.mk.in
* re-work spacing in README.md to pass 'make whitespace' while still
aligning properly with github markdown rendering
* make example configuration and README documentation match
* add rlm_couchbase and short description to raddb/README.rst
'new modules' section