Adds ua method for providing your own UserAgent.

Also documents previously secret "url" attribute.

I'm working with a local copy of 02packages.details.txt which is created by OrePAN.  HTTP::Tiny won't fetch files and this seems easier than firing up my own Plack server just to be able to read a file.  ;)

I added a workaround for caching, since I was always getting a 200 even when the file hadn't changed.  If you can use this, feel free to bastardize it to your heart's content!