Bowercopy update jquery-turtle, ZeroClipboard, require.js.

Jquery-turtle brings in miscellaneous library fixes, plus the Webcam
class.

ZeroClipboard is upgraded to 2.0.1, which has a minor API change.
Code updated to match the new API, and Gruntfile updated to reflect
the new dist layout.

No major change in require.js.
Bowercopy update: latest jquery-turtle.
Discourage too-long usernames.

Usernames longer than 20 chars look like they aren't mnemonic,
so discourage these longer names.

Also, discourage names that look like possible email addresses
(ending with a common domain suffix).