Add custom window constructor

[idanrozin]

So, related to issue #124 I have opened a PR for whomever is interested. hope it helps and someone will take it to review and merge :)

regarding the tests. I know it looks awful in the diff view but what I essentialy did is just to create another test suite which runs the same tests but with the new constructor edition. so i needed to extract the funcitons logic outside and use closure on some. but basically this is the structure:

before each (the only change here is the instantiation of StackdriverErrorReporter and window which is done inside the tests suites itself.
first suite -> same logic
second suite -> same logic just with new StackdriverErrorReporter(customWindow)

Thanks!

[google-cla]

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[steren]

Thanks for the PR @idanrozin, any reason why you closed it? Signign the CLA is standard to contribute to Google-own code.

[steren]

Ah sorry I see #126