406 Not acceptable on OS X
I am not sure why exactly but every simple request on OS X from `urlopen` and `urlretrieve` results in 406 errors.

OS X 10.10.4
Python 2.7.6

After some fiddling I realised, that it's related to missing the `User-Agent` header bit or the accepted mime type stuff. I quickly fixed it by overwriting the global opener instance with one using a sensible `User-Agent` value.

The commit can be found here: https://github.com/trevex/glad/commit/e5815d1b950b0c242e052624c94b6b5c4498b2bb

I'll test it later on my desktop pc running Ubuntu to check whether it breaks anything. I am not an experienced python developer, so let me know if I should send a pull request or you'll fix this in a more idiomatic or cleaner way...
