Initial pull request

This pull request simply adds the source code from [GRR](https://github.com/google/grr).
To be exact, it corresponds to this version of GRR: https://github.com/google/grr/tree/1df883fb3504b259a25d149311074230f2a91302.

FYI, the documentation for GRR is hosted here: https://github.com/google/grr-doc