Callbacks: Disabling a callback should prevent firing

Thanks to @TheDistantSea for the report!

Fixes gh-1790
Closes gh-1643
(cherry picked from commit bc1cb122db30be034b3da84ee2546c86e2a5268f)
Event: Fully clean up events in unit test

Closes gh-1685
(cherry picked from commit 4467ed606ab0a9b28ed495e682576910937fa5e0)
Manipulation: improve test for data-URI

Some environments do not support data-uri in "src" attribute of script element.
Mitigate it with equality assertion
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