Improved bidirectional databinding, themes support and added attributes

Hello!
I have added/improved a few things for this element:
- better handling of default "value" : in case the element is used without any content (ie <ace-element></ace-element>) , but with some code passed through the "value" attribute, the contents of the editor are filled correctly
- added "themes" attribute: this allows the element to make all the available themes visible from the "outside": can be quite handy for example if you want a dropdown with all available themes to choose from
- exposed the "showInvisibles" parameter of Ace as attribute (with correct attribute change handler).

Hopefully I did not make too much of a mess otherwise (works fine on my end). Thanks for the great groundwork !