Add network.ParsePortRangePort.
ParsePortRangePorts -> ParsePortRange.
Add network.ParsePortRanges.
Add a round-trip test for PortRange strings.
Move ParsePortRange into portrange.go.
Merge pull request #1298 from ericsnowcurrently/network-parse-port-range

Add helpers for parsing port range strings.

We'll be using this in the GCE provider.

(Review request: http://reviews.vapour.ws/r/612/)