JSONP: Accept anything as a 'callback'

As discussed there: https://github.com/buildbot/buildbot/commit/4985d3f7c5b9c7be1968804cade7b62fd8db45d8.

The previous regular expression was too strict, and from multiple sources on the web, it seems that we can accept any arbitrary text over there ...

http://bob.ippoli.to/archives/2005/12/05/remote-json-jsonp/
http://www.json-p.org/