Add with and only_with chain to be_mounted matcher

With this chain, you can test a given path mounted with correct attributes like this.


```ruby
describe '/' do
  it { should_not be_mounted.with( :type => 'ext4', :options => { :rw => true } ) }
end

describe valid_mount do
  it do
    should be_mounted.only_with(
      :device  => '/dev/mapper/VolGroup-lv_root',
      :type    => 'ext4',
      :options => {
        :rw   => true,
      }
    )
  end
end
```

``only_with`` needs all attributes of the mounted path.