Detect source files generated by the Protocol Buffer compiler

Detects Python, C++ and Java code generated by the Protocol Buffer compiler (https://developers.google.com/protocol-buffers/).

The sample generated files were generated from the input:

```
package persons;

message Person {
  required string name = 1;
}
````

Let me know if that should be checked in too (though #392 is still open about detecting .proto files, and has its own sample)