Type spec: Logging vnode

Started with fixing the type specs. Dialyzer doesn’t give a warning any
more.
However, the types can be further refined.
Simplifying the API code

Error message with reason.
Polishin

Small changes to the type specs
Started with fixing the API

Added type spec and short description. Still quite a few open questions…
Merge branch 'master' into specs_fix

Conflicts:
	src/logging_vnode.erl
Merge pull request #52 from SyncFree/specs_fix

Specs fix