Table#strings

Added some methods to the `UiaDll` to get headers and values back from a table.  Then refactored the `Table#strings` method as well.  As an aside, this pull request is setting up for something I want to be able to do with `Table#row`.  I would like to be able to have this:

```ruby
  table.headers # => ["Name", "Date of birth", "State"]

  first_row = table.row(:index => 0)
  row.name # => "John Doe"
  row.date_of_birth  # => "12/15/1967"
  row.state # => "FL"
```