Added support for multiple pants_support_baseurls.

Changed pants_support_baseurl to a list. Pants now goes through each url in the list from first to last, downloading the first binary with a valid url (and then breaking). This way, pants users can set their own support repo online and add it to the beginning of the list, so that their repo will be queried before the default repo on pantsbuild.github.io.
It happens exactly once (unless the binary doesn't exist on any baseurl, in which case a TaskError is raised); I have unit tests that check that. I 'with' statement would be more appropriate, however the called function just returns a generator, not a closeable object. I could refactor that.