Fix dom stripping

The Oracle comparators are now gone because

1. They do not adhere to the plug-in architecture.
2. They are not configurable enough
3. They were not used consistently throughout the application
4. They weren't thread safe

The new DOM strippers offer much more flexibility. Here's how they'll work:
The idea is that you define zero or more of these strippers. They do not have to produce a valid DOM, but they're there so you can influence how DOMs are compared. StateVertexes are compared on the equals function of these stripped doms so the more alike you make them, the less states you'll have at the end of your crawl.

Here's an example of a stripper:
```java
/ **
* Filter nodes with attributes this key and value.
 *
 * @param key   The key of the node.
 * @param value The attribute of the node.
 */
public ByAttributeStripper(String key, String value) {
	this.key = key;
	this.value = value;
}

@Override
public Document apply(Document doc) {
	Elements elementsByAttribute;
	if (value == null) {
		elementsByAttribute = doc.getElementsByAttribute(key);
	}
	else {
		elementsByAttribute = doc.getElementsByAttributeValue(key, value);
	}
	for (Element element : elementsByAttribute) {
		element.remove();
	}
	return doc;
}
```

Also fixed

- a bug where the iFrame contents was crawled when it was configured not to.
- a bug where iFrame content was used for DOM comparison while crawljax was instructed not to crawl frames.

Removed:

- Test-utils because it wasn't used correctly. The parts that were used are now in core.
- Filter attributes, because those are now present in the new DOM strippers.