Implemented OntologyLink Addition Mapping Step.

Quite a few comments are added, please read them and let me know if my
understanding and implementation are correct.

In addition, freebase API URL is not working, please read comments in
"FreeBaseLinker.java".
Added OntologyLinkInstance, which extends SchemaInstance.

OntologyLinkInstance not only tracks "content" (XML tags), but also
"value" (actual value used by AttributeInstance). This is done so that
when an attribute is promoted to be an ontology schema, but later
demoted back to be an attribute due to one-to-one relation with parent
schema (schema flattening), the "value" of the attribute instance is not
lost.
FIXME comments added.
FIXME content re-committed.
Merge pull request #11 from yaoeric/devel/refactor

Implemented OntologyLink Addition Mapping Step.