Download protobuf distribution instead of using the one available

[pires]

In a multi-developer & multi-platform environment, one should take in mind that protoc location is a mutable value.

Since this plug-in allows you to define protoc path in the POM, this is somewhat covered. But would be great if the plug-in could simply download the protoc distribution (depending on the version specified in the POM) and make it available to the user in ~/.protobuf_dist. It would only take the time to download the first time but would then be available to every developer, no matter what OS he'd be running.