Remove IE7 / IE8 support

[kaljak]

Lots of libraries have already dropped the support of IE7 / IE8 and I think so should do this library.

e.g. "attachEvent" or "domLoaded" function

[akomm]

Many even dropped 9 & 10 support.

[kobeaerts]

Yeah, we're 2017. Time to drop everything below IE11 🎉

[marcj]

well, no need for that since IE is actually the only browser that supports this native :D

[kaljak]

To make it more explicit I created this commit: kaljak@bc7d560

All workarounds, that are only needed for IE8 and maybe Firefox < 3.6 are removed. I will make a pull request if you know now what I mean :)