Split mvn property protobuf.home property into protobuf.include and protobuf.bin

This allows mvn to reference the homebrew installation, rather than a relative one by setting the two properties in your ~/.m2/settings.xml file. This should work exactly the same by default.

```xml
      <properties>
        <!-- Properties to set if you installed protobuf from homebrew -->
        <protobuf.bin>/usr/local/bin</protobuf.bin>
        <protobuf.include>/usr/local/include</protobuf.include>
      </properties>
```