New equal?-method to compare with expected data

As a developer I would like to compare the data of a `CornerStones::Table` with the data of a `Cucumber::Ast::Table`.

At the moment I have to do the following:

```ruby
  table = CornerStones::Table.new("table_selector")

  expected_data = expected_results.hashes

  actual_data = table.rows.map {|row| row.attributes.reject {|key, _value| !expected_data.first.has_key?(key)}}

  actual_data.should == expected_data
```

With this PR I can do:

```ruby
  table = CornerStones::Table.new("table_selector")

  expected_data = expected_results.hashes

  table.equals?(expected_data).should == true
``