Start supervisor children based on the `pool' setting

In the future, this will allow Katja processes to be easily "poolable",
because you can essentially decide which processes should be
started/monitored by Katja directly and which ones should be start in a
pool. The pooling library does not matter.
Merge branch 'master' into development
Mention `pool' in README
Add API to either start registered or unregistered processes
All `katja' methods now optionally take a pid

All methods inside the `katja' module (public API) now optionally
take a pid as their first argument. This makes it possible to
pool `katja_metrics' and `katja_queries' processes.
Add tests where a pid is explicitly passed

Add tests for functions that now directly a pid.
Add test katja_sup
Fix Dialyzer warnings
Mention pooling support in README
Bump version
Update overview.edoc