Add a contains() function to subnets.

This implements the feature request in #18.  However, I opted for hanging a function off of the subnet() return value, rather than a top-level function called subnetContains.  It could obviously be switched around, but I personally favor this approach.  I don't know if others are expecting subnet() to never return an object with a function as a member, which would be the only downside I can see to this method.

This includes tests and an example of usage in the README.md.
80 column limit?