Add puppet:// type path support for certificate, private_key and chain

Closes #19

> As a puppet user,
> I want to use puppet:// style path for certificate, private_key and chain parameters
> So that I can omit to write a file resources for those.
> 
> Since existing specs won't work for my environment, I could not write spec tests for this PR.
> Does anyone help me out how setting up specs could be run?

Added docs and integration tests.