bitcoin uri handling

This adds support for bitcoin URI handling. To manually test, first create a wallet and accept the URI handling request from your browser. After that, close the wallet (this won't be required if we implement this: https://github.com/bitpay/copay/issues/807) and click a bitcoin URI (you can find one here: http://www.proofofexistence.com/detail/fd1ba09f0f106cfde209e6abc435151150d2d3ace7e100ee17316618d51415ae) 
fixes https://github.com/bitpay/copay/issues/757


The API should be able to process a bitcoin URI. It won't "handle" it the way a web app might receive an event that such a URI has been clicked, but you give it a bitcoin URI and ask you to sign a transaction to that address of that amount, and, if the bitcoin URI is a BIP70 URI, it retrieves the request.