Provide a detach shim for jQuery prior to 1.4.

Flot 0.8 still officially supports jQuery 1.2.6 and higher, so we can’t
assume the existence of detach.  This will go away in 0.9, where the
minimum jQuery version is 1.4.4.

Fixes #1240.
Merge pull request #1286 from dnschnur/jquery-detach-fix

Provide a detach shim for jQuery prior to 1.4.