Added cancel label and cancel callback function

The label of the cancel button and a custom optional callback function
can now be specified. Works just like the success button.
Changed cancel button in default header template

The cancel button in the default template header i snow visible by
default and mimics the the look of the bootstrap example modal.
Merge pull request #9 from johnparn/test

Customizable cancel label and callback function and some minor fix of cancel button in the default modal header template.