2-way binding, efficient data mutations, and bindable selection
- Templates should now 2-way bind to `model.*` from user data array (breaking change)
- Handle data array splices more efficiently: initialize never re-assign the physical data array that drives the template repeat; only lazily add to it when adding, and hide already generated items when removing
- Make selection bindable, and handle switching between multi and non-multi
Test branch for testing gulp-web-component-tester integration.
Align with polymer-test-tools
Merge branch 'wc-tester' into 2way-improved

Conflicts:
	core-list.css
Fix template binding delegate.
Loosen the focus gambit to ease ability to test (no impact on actual use).
Add real tests.
Update tests to latest web-component-tester.
Add support for iOS8 scroll events.
Update bower ignore to only include production files.
Merge pull request #30 from Polymer/2way-improved

2-way binding, bindable selection, and efficiency improvements - fixes #2 #7 #9 #13 #15