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
This is not a `// Supports: ` comment so rules from jquery/contribute.jquery.org#95 probably wouldn't apply. We should get such a comment here, though.