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
Actually, I'm not sure, thanks for making me look into that. For context, see the discussion at #1597; some comments mention IE8. @markelog, you said at https://github.com/jquery/jquery/pull/1597#issuecomment-46289967 you couldn't reproduce it in IE8; do you think it might not be needed anymore?