Add SBT version to example path for credentials file

As of sbt 0.13, the credentials won't be found if they aren't within a version directory.
Docs: use programmatic version in global base directory
Docs: fix typo.  Fixes #951.
simplified settings core example (as per `util/collection` `SettingsExample`)
Fix type error in example code

Fix #971.

Note that this is no complete fix, because this solution uses an
operator which is not any more described in the guide for 0.13.
Docs: correct substitution syntax when preceding character is symbol.  Fixes #970.
resubmit of #983