New JSON API.
Bumped version to 1.13.0, so I don't forget it before the release.
Supprt stdClass objects in nativeToDatum conversion (Implements #64)
Added r\random() and changes(). Fixed index creation range condition in test cases.
Adapted r\Cursor to work with change feeds.
Added test for eqJoin taking a function as its first argument.
Added r\http().
Fix token handling for change feed responses.
Minor efficiency improvements in Datum to JSON conversion.
Re-introduced use of r\json in write queries. It yields much better performance.
Revert "Minor efficiency improvements in Datum to JSON conversion."
This broke handling of implicit vars.

This reverts commit 5b60b48840166cb8088a5c731ddb5e2b6b14832a.