Correctly register _z in momentProperties

momentProperties changed its semantics in 2.8.1, so make sure moment-timezone
works with both the old and the new behavior.
Forward all moment.utc arguments from wrapper
Merge pull request #110 from ichernev/fix-moment-props

Correctly register _z in momentProperties
Merge pull request #111 from ichernev/fix-zone-wrap

Forward all moment.utc arguments from wrapper
Support 2.8.1 with the _z and _a properties
Fix add/subtract argument order
Version 0.2.1