Added getClasses

getClasses returns the list of classes of the given element. Changes to
it will not be reflected in browsers that do not support classList.
Clean up
findFieldByType and findFieldByName

- renamed findField to findFieldByType
- added new findFieldByName
Custom validation improvements

- These changes allows for multiple custom validations set in the same
field.
- The paramaters are set inline and not in a separate attribute.
- Added support for referencing fields by name.
- The arguments can be passed in quotes.