#21895 #23636 added url_download method and sg connection studio hook.

- Adds a new method called sgtk.util.download_url() which makes it easy
to download for example images. This method takes into account the proxy
settings used by the shotgun API when it connects.
- Adds a studio level hook which lets you override the connection
settings retrieved from the shotgun and app_store configs. This is
useful if you for example have a dynamic proxy which changes depending
on some external set of circumstances (e.g. your physical location).