Add have_ipv4_address matcher to interface type.

Add `have_ipv4_address` matcher to `interface` type.
I tested this implementation on Ubuntu 12.04.

```ruby
describe interface('eth0') do
  it { should have_ipv4_address("192.168.10.10") }
  it { should have_ipv4_address("192.168.10.10/24") }
end
```