R documentation with roxygen2

[hetong007]

Currently the functions are documented in the style of roxygen2, while no actual docs/NAMESPACE file generated by the package.

Hopefully this issue tracks the progress on automatic documentation generation with roxygen2 for the R package.

[tqchen]

cc @thirdwing I think we need to resolve some of the issues mentioned at #242

[tqchen]

The roxygen style generator is checked in #296