No longer failing to detect dates when test frameworks overwrite the Date constructor (poorly)

Specifically, SinonJS overwrites the Date constructor with a function that produces date objects
whose constructor property is the original Date constructor, while Date currently refers to the Sinon wrapper.

That hullabaloo makes a.constructor !== Date end up true when it should be false.
Merge pull request #80 from elarkin/master

TimezoneJS should be a little bit more robust when test frameworks wrap the Date constructor.