(PDOC-3) Rename face to strings instead of yardoc

Since we don't want the name of the tool to reflect the fact that
it is using yard internally (this is an implementation detail), rename
the face to `strings`. Now when one wishes to generate documentation,
`puppet strings` will be used rather than `puppet yardoc`.
(PDOC-3) Add puppetlabs directory

In order to keep with conventions around directory structure, add
a puppetlabs directory and rename modules accordingly.
(PDOC-3) Rename FutureParser and ParserFunctions

Prior to this commit, the handlers and code objects which dealt with
puppet functions were referred to as `FutureParserFunctions` and
`ParseFunctions`. This naming convention will not make very much
sense going forward when the future parser becomes the primary
parser. Therefor, rename `FutureParserFunctions` to `Puppet4xFunctions`
and `ParserFunctions` to `Puppet3xFunctions`.
Merge pull request #4 from hkenney/PDOC-3_refactor-prototype-code

(PDOC-3) Rename face to strings instead of yardoc