Add async.nextTick browser fallback to setImmediate

This adds the nextTick fallback for browsers to support
the proposed setImmediate interface, which is similar
to process.nextTick in browsers that support it, or on
sites that implement a shim for setImmediate before
loading async.
Merge remote branch 'dougwilson/feature/setImmediate-support'

Conflicts:
	dist/async.min.js
replace process.nextTick call with async.nextTick and increase timeouts for some tests