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've changed it to `IE<9` since we use this form in many places anyway but, as @markelog mentioned in https://github.com/jquery/contribute.jquery.org/issues/95#issuecomment-66111865, I don't want it to be a blocker, I'd like to merge this PR today or tomorrow.