add factories for `path` and `url`

This adds some factories for generating random paths and urls.  The
`path` factory is just a more convenient alias for `dir_path`.  The
`url` factory builds on `path`, adding a leading `/` and an optional
host value.

These are just additions to make the factories more expressive.

Closes #189.

@jcredding this is ready for review.  I chose not to auto-require the factory module as it is an optional add-on and I kinda don't want to impose it on you.  I kinda want requiring `'assert'` to be about just adding the stuff needed to run a test and display its results.  Same reasoning for the upcoming `stub` module/add-on.

Let me know your thoughts - I could go either way on this but I'm leaning towards to auto-requiring it.