Allow to display a flash message on lock.show

[glena]

Closes #637

Added a flashMessage option to the show method.

Showing an error flash message:

lock.show({
  flashMessage:{
    type: 'error',
    text: 'error message'
  }
});

Showing a success error message:

lock.show({
  flashMessage:{
    type: 'success',
    text: 'success message'
  }
});

Showing errors in redirect mode:

lock.on("authorization_error", function(error) {

  lock.show({
    flashMessage:{
      type: 'error',
      text: error.error_description
    }
  });

});