Auto merge of #3485 - sfackler:useragent, r=alexcrichton
Auto merge of #3485 - sfackler:useragent, r=alexcrichton

Set a user agent with the version when talking to the registry

This can be used to track what versions of Cargo are being used in the
wild, which can be very useful.

r? @alexcrichton

durka/cargo,tshepang/cargo,tshepang/cargo,nipunn1313/cargo,rust-lang/cargo,eminence/cargo,tsurai/cargo,achanda/cargo,rust-lang/cargo,tsurai/cargo,tsurai/cargo,vojtechkral/cargo,nipunn1313/cargo,eminence/cargo,eminence/cargo,durka/cargo,sbeckeriv/cargo,rustyhorde/cargo,vojtechkral/cargo,rustyhorde/cargo,rust-lang/cargo,tsurai/cargo,achanda/cargo,nipunn1313/cargo,vojtechkral/cargo,rust-lang/cargo,sbeckeriv/cargo,tsurai/cargo,achanda/cargo,sbeckeriv/cargo,durka/cargo,durka/cargo,achanda/cargo,tshepang/cargo,sbeckeriv/cargo,vojtechkral/cargo,rustyhorde/cargo,nipunn1313/cargo,tshepang/cargo,rustyhorde/cargo,eminence/cargo