Bug 988079 - [Calendar] day view sometimes adds duplicated days to the DOM r=gaye

second try! changed way more stuff; removed logic related to multiple frames (now it always add/remove elements from DOM as needed), and added integration tests to make sure swipe actually changes dates. - to avoid regressions.

removed all code/tests that wasn't needed anymore.

PS: I still need to run the tests 30+ times on travis to make sure we don't have intermittent failures before merging this patch.