[WIP] Adding assert for type and tag

Implements `assertWrappedElement` to check for correct wrapped element in typified elements.

Closes #53 
How does the final error message look like? It seems that will be like:

```
Wrapped element in {ClassName} must {AttributeName}} be "{AllowedAttributeValues}", but attribute of "{AttributeValue}" tag given
```
Somehow it doesn't sound very correct.