Add logElementTreeJSON() and friends

array unique() and object getMethods
improve array and boolean assertion diagnostics
support options for screenCapture and json logging on test failure
logDeviceInfo()
Add argument for assertScreenMatchesImageNamed to set diff threshold.
Merge pull request #56 from kconnor/master

logElementTreeJSON, etc
Refactor image_asserter.
Merge pull request #57 from giginet/image_assertion_threshold

Add argument for assertScreenMatchesImageNamed to set diff threshold.