Introduce Napybara::Element.

Will eventually replace DSL.
Replace DSL with Element.
Allow elements to be found with id.
Add method_name as find option.
Rename find to finder.
Alias get from capybara element.

Fixes rubocop warning: Use attr_reader to define trivial reader methods.
Add active support string extensions.
Let finder add 'all' method to element.
Let finder add has_<record>?(record) method.
Let finder add has_no_<record>? method to element.
Refactor Element::Selector.
Update readme.
Bump version to 0.3.0.