Changed some stuff
Tighten up interface, don't leak Uri implementation

So as not to obligate client code to import com.netaporter.uri.dsl to
get implicit conversions from strings to Uri's, companion object
factories now generally assume they will be getting strings, whereas we
use Uri internally
Add a high-level acceptance test
Switch to using an Enumeration for changefreq

Turns out I kinda like the namespacing of it
Update version and README for newness