Proxy support for MapBox authorization

[ansis]

The browser TileMill is running in needs to be aware of the proxy in order for authorization to work.

The real fix would be to pass it as an option to topcube, and get topcube to set an environment variable within that process. The browser should pick up on it. Should be fairly simple to do.

If we don't get around to fixing it, we should add a dialog informing the user of the workaround (running TileMill in a browser to authorize their account) when they try to authorize.

[springmeyer]

@Aibram - from the sounds of this ticket the only actionable thing for 0.10 might be to add a warning?

[ansis]

Yeah, I'll add a warning and a link to authorize in a browser.

[ansis]

Opened upstream ticket at creationix/topcube#16

[ansis]

Revert 2f2dc84 when properly fixed