plugins: implement Munin::Plugin::HTTP

With this module, it's easier to make all the HTTP-based plugins
behave the same way (at least those written in Perl), in particular
responding to the same timeout and providing the same user agent on
requests (or the same user agent schema, at least).
plugins: implement standard variables for HTTP authentication.

Instead of having each plugin handling authentication in a different
way (some with variables, others with URIs), make available standard
variables, and provide an authentication callback to libwww-perl
directly in our module.
Merge pull request #48 from Flameeyes/plugin-http

HTTP plugin helper