Enable Sauce runs on IE on an opt-in basis

[rsimha]

This PR does the following:

1. Adds IE 11 to the mix of browsers used on sauce labs for testing
2. Disables testing on IE 11 by default
3. Adds functions enableIe and ifIe to allow test writers to opt in to testing on IE
4. Updates all integration tests that use just describe( with describe().configure().run(, so that the new default TestConfig is picked up

In the new world:

• describe('foo') is no longer meant to be used for integration tests (if used, it will include IE)
• describe().configure().run() will run tests on all browsers except IE
• describe().configure().enableIe().run() will run tests on all browsers including IE
• describe().configure().ifIe().run() will run tests only on IE

Fixes #10731

[cramforce]

Lets opt in at least one test.

[rsimha]

Opted in amp-bind tests.

[rsimha]

@choumx, we now have a working solution for sauce labs testing on IE. See PR description for details.

I've gone ahead and enabled a few integration tests like amp-bind on IE to see if they pass or not.

[rsimha]

@choumx, @cramforce, we now have IE 11 in our browser matrix. I've enabled a bunch of tests on IE 11: see https://travis-ci.org/ampproject/amphtml/jobs/267750202

This PR is ready for review.

Comments have been addressed.

[rsimha]

All comments are addressed, and we now have more than a dozen tests running and passing on IE 11. Merging this PR.