Pca9685 support

Hello - as discussed in [https://github.com/raspberry-sharp/raspberry-sharp-io/issues/8] here is my pull request for Pca9685 support.

I have:
- Fixed namespace/casing
- Used NDesk nuget package
- Added gitignore for packages folder (best not to stash binaries in git, let nuget do package restore
- Updated the readme.md to note the Pca9685 support

I haven't:
- Added Common.Logging yet. It might be a few weeks before I get around to this so figured it would be best to submit the PR while I was still looking at this.