Added datepicker_use_button option

Note that the new option does not use the dp_ prefix since it's not a datepicker javascript option. It is used in the Twig template to control the HTML, which then controls whether a button is used for the datepicker or not.
Added test for presence of datepicker_use_button
Implemented datepicker_use_button option

If datepicker_use_button is false then the extra HTML elements for the activation button are not rendered, and the datepicker is activated by clicking on the input field instead.
Updated to work when datepicker_use_button is not set
Use the attr parameter to pass format options through to date widget when not using a separate button to activate the datepicker widget
Added documentation for dp_* options and new datepicker_use_button DatePicker options
Fixed typo in doc
Merge pull request #90 from caponica/datepicker_optional_button

Added option to use datepicker without a separate button / icon