Refactors

Refactors the schema generator into a generic pipeline of mapping steps
can be configured at runtime. This commit contains only the basic schema
extraction step as a proof of concept.

In the new version the generator will be called as:
```java
new MappingGenerator()
    .addStep(new BasicSchemaExtraction(1000))
    ...
    .generateMapping(rootDoc.getDocumentElement(), "http://www.linkedct.org/0.1#");
```