Spoofed 'User-Agent' in header if proxy is used during install to prevent issues with corporate firewalls

Ran into similar issues in node.js with `bower` [but could be fixed with `.bowerrc`]. Basically, with corporate firewalls (and doing research online, I'm not the only one), `http://` requests with a bad/blank/unknown `User-Agent` string are automatically rejected. As such, install never happens.

My team uses this package [as part of grunt-mocha], so installing when on the network (as opposed to getting off the network temporarily every time we wanted to `npm install` our scaffold) was an elusive, but important function. After similar issues with the latest version of `bower` [https://github.com/bower/bower/issues/698], I became intimately aware of these issues. 

Contributing this pull to help people in similar situations.

I spoofed `curl` since this was pretty generic.