add BIP70 protobuf features in new PayPro lib file

BIP70 uses protobufs to for the message format. This PR adds support for all the standard BIP70 message formats using a library called protobuf.js (protobufjs on npm) which works in both node and the browser, with optimizations for node (buffers rather than arraybuffers). Since protobufjs adds hundreds of kilobytes to the bundle, it is includede in "all", but not "main". You have to build "all" to get this.

No BIP70 in the browser: <code>node browser/build.js --includemain</code>

BIP70 in the browser: <code>node browser/build.js --includeall</code>