add zfs matcher

I've added zfs matcher.

Usage

describe 'rpool' do
  # ZFS 'rpool' should exist
  it { should be_zfs }
  # property 'mountpoint' of ZFS 'rpool' should be '/rpool'
  it { should be_zfs('/rpool').property('mountpoint') }
  # property 'quota' of ZFS 'rpool' should be 'none'
  it { should be_zfs('none').property('quota') }
  # property 'compression' of ZFS 'rpool' should be 'off'
  it { should be_zfs('off').property('compression')}
end