=> Rebar 2.5.0
Fix google charts on ssl (Fixes #81)
Fix typo in #custom docs

Also expand a bit on the 'tag' semantics
Session handler more resilient of malformed state
Add to wf:to_existing_atom

With docs
Merge branch 'master' of github.com:nitrogen/nitrogen_core
Merge branch 'master' into ws
Add typespec to wf:to_existing_atom