Add `externalIncludePath` to `cleanFiles`
Slight reordering of task dependencies.

`includePaths` doesn't depend on `unpackDependencies` anymore.
Building the list of include directories shouldn't depend on unpacking
the dependent protobuf files.

The `generate` task depends on `unpackDependencies` now.
Nicer debug log for listing the compiled protobuf files
Use sourceDirectories for collecting the protobuf files to be compiled.

By using sourceDirectories, a project has the flexibility to not only compile its own protobuf file,
 but also the protobuf files from dependencies, by adding this to the settings:

 sourceDirectories in protobufConfig <+= (externalIncludePath in protobufConfig) apply (identity(_))
Update README for the latest changes.
Set version to 0.3.3