Allow text changes via data attributes...

... and allow to change the "OK"-button (See Issue #18)

This patch allows to set the text fields via the data-attributes of html-tags. Now it is possible to customize the message for each confirm-dialog, for example one could now could write "Do you really want to delete user XY?" instead of "Really delete that user?"

As a second change I added the possibility to change the class of the "OK"-button. I later found out this has been requested in Issue #18, so this also fixes #18.