added be_routed matcher

Hi. I have added the matcher of routing table.

#### examples
```ruby
describe '33.33.33.0' do
  it { should be_routed }
end
```

```ruby
describe '33.33.33.0' do
  it { should be_routed.with( :iface => 'eth1' ) }
end
```

```ruby
describe '33.33.33.0' do
  it { should be_routed.with(
    :iface => 'eth1',
    :gateway => '0.0.0.0',
    :flags => 'U',
    :genmask => '255.255.255.0'
  ) }
end
```

Thank you confirmation.