Add user-agent to API requests

[Matt Raible]

Code from AngularJS SDK: https://github.com/stormpath/stormpath-sdk-angularjs/blob/1b4cbe9bc77577300b108467f3016114a49613d4/src/module.js#L226

See notes about it needing to be same domain, if we add it on cross-domain requests it’s going to fail if the remote domain isn’t allowing the custom header.