Misc: Drop support for older browsers; update support comments

That includes IE<8, Opera 12.x, Firefox<29, Safari<6.0 and some hacks
for old Blackberry.

Fixes gh-1836
Fixes gh-1701
Refs gh-1815
Refs gh-1820
I think we really need to document style of these comments otherwise it's subjective opinions that do not provide a clear guidance - https://github.com/jquery/contribute.jquery.org/issues/95