Adds subnet support for is_in?

Changed implementation for is_in? so subnets are supported.
Added test.
Comparing network and broadcast addresses is also much faster (a quick
benchmark shows a huge improvement). Hope I'm not missing something.
Fixes #9
Adds more tests for is_in?

Fixed previous test and added more so we have positif and negative
assertions on single IPs (/32) and larger subnets
Merge pull request #10 from leucos/master

Adds subnet support for #is_in?