Refactored accessor methods
Use deprecate for global export deprecation

It looks like this in the root Node.js scope is NOT the global scope. So
rename global -> global_scope and fall back to 'global' in case it exists.
Fix addOrSubtract to properly handle updateOffset
Add logic in zone to handle impossible keepTime-s
Fix startOf issues across DST with updateOffset
Make jshint happy
Merge pull request #1564 from ichernev/fix-zone-madness

@changelog
@section bugfix
@description Simplify zone handling