Elements present async

[kbingman]

Async added back to snoop to speed things up.

Status: Ready for Review

Reviewers: @scalvert

Changes

• Puts the async module back in into Snoop
• beefs up the tests
• returns the number of missing items in the callback

  How to test-drive this PR

• run the tests npm test
• install this branch in a project (Bose, for instance)
• run the nightwatch tests

[scalvert]

:+1: