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.
If there are no baseurls configured it looks to me like the code is going to keep on going and eventually raise a TaskError.  But the real error is that there are no baseurls configured. If I'm reading the code correctly, I'd rather raise an exception here with a more descriptive error.