Implement the basename() function to stdlib.

I always wondered why basename() wasn't in stdlib since dirname() was.

We ended up writing our own locallib module to implement the basename() function to keep from having to write inline ruby code. We were working on a nfs mounting module and wanted to default to the basename of the filer's exported path to be the default mount point. This new function gives us that feature.

This is just a modification of the dirname.rb file where all references to dirname were replaced with basename. This same implementation in our locallib works fine.

I have also updated the spec/ files to behave as I think they should. 

Thanks for considering this new function.
can you get rid of this, and instead use `:arity`, please?
see, https://github.com/puppetlabs/puppetlabs-stdlib/pull/249