Roxygenize

In this branch on my fork of `rmongodb`, I converted the package to use `roxygen` (https://github.com/klutometis/roxygen) for generating documentation, keeping docs in sync with the code, generating the `NAMESPACE` file, and so on.  This works really great with RStudio for building the package, it handles much of the tedium for you.

I had a few other things I wanted to add to the package, but first this change would make hacking on the package easier.