Fix get interface

This patch piggybacks on #133 and provides a better interface to create... interfaces.

Please review and merge #133 first, presuming it passes muster.
I'm wondering if we actually need an options for this? As far as I can tell, you either want to provide the bridge name and expect Docker to use it or fail, *or* you leave it up to Docker and then the code should do its best to create a new bridge regardless of its name.

In the end, I'm still not super confortable with the network name being so closely tied to the interface name. I'd rather be able to name my network "horse_battery_staple" if I want to, and either specify a bridge name to use, or leave it to Docker to create a bridge under whatever name it wants (which is where the logic of appending an integer to a default prefix should probably come into play).