Added support for building wireshark HEAD from svn directly.

I modified the wireshark.rb Recipe to pull HEAD from SVN and build it. This is the first time I have modified a recipe, and I am curious if I did something correctly.

When building from svn it is necessary to run autoconf against the source. I check to see if the user is requesting a HEAD build like this:

if build.include? 'HEAD'

Is that the 'correct' way to detect a HEAD brew?