Add a possibility to use auto and initial in setHeight calls

In order to make `setHeight` consistent with CSS `height` property, it's better to add support of 'auto' and 'initial' values as arguments in `setHeight`.

Requirements:

• `setHeight('auto')` - enables auto height adjustment;
• `setHeight('initial')` - sets initial iframe height taken from the configuration (and disables automatic mode);
• `setHeight(<pixels>)` - sets custom iframe height (and disables automatic mode);
• add resize event listener in automatic mode to react correctly on image loading events;
• possibly remove `updateHeight` public method since the problem with images is fixed by the resize event.
• remove `autoUpdateHeight` public method since it's possible to pass `auto` into `setHeight`.