consistent naming
Change cookbook name to 'r'.

This commits standardizes the naming within the cookbook to use a
lowercase 'r'.  Using 'r' rather than 'r-project' provides more
sensible resource and attribute names.  Using a lowercase 'r' ensures
that resource names have more Ruby-esque names (r_package as opposed
to R_package).

The cookbook is still uploaded to the community site as 'r-project'
since the 'R' name is already taken.

Merge remote-tracking branch 'taqtiqa-cookbooks/feature/rename'