Added preliminary support for asynchronous testing.

Conflicts:
	lib/screw.builder.js (added closing tags for IE Support)
Asynchronous tests are now run sequentially instead of parallel.
Increased waitFor-polling frequency.
Fixed BUG: When no async jobs left, run should trigger 'passed' (previously it didn't)
Improved detection of asyncronous it-specs.
Updated the README (added documentation on asynchronous testing)