Add Pre-Close Callback/Confirm Close

Add the ability to specify a callback that is called before actually
closing the dialog to allow the close to be canceled.  For example, if
the dialog contains some sort of editing work and the user accidently
presses escape the callback can confirm with the user that they want to
exit and return false to prevent the dialog from being closed.
Pre-Close Callback/Conform Close Example

Update example's index.html to use pre-close callback.
Merge https://github.com/likeastore/ngDialog into confirm-close
Pre-close with nested confirm

Update to allow for a nested confirm within a confirm.  Previously was
using window.confirm to block execution flow, but now uses the returned
promise to determine whether to keep the parent dialog open based on the
nested confirm's resolve/reject.  Also changed the pre-close to not be
called when calling a confirm dialog's confirm method.
Update README.md

Add details and examples about the preCloseCallback option.
Merge pull request #75 from curts/confirm-close

pre-close callback feature