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.
Merge pull request #190 from redding/kr-factory

add factories for `path` and `url`