proposal to refactor "extracted" flag to accommodate additional states

In an effort to identify potential backwards-incompatible changes that may need to be applied in preparation for our milestone 1 release, I keyed in on the `extracted` flag. The intent of the `extracted` flag is to identify a conclusion as "extracted" from a single source. I think this is an important state to identify, but I'm wondering if there are more than just two "states" of a conclusion--I'm thinking there are three: "extracted," "hypothetical" (the conclusion is working hypothesis), and "accepted" (the conclusion has been formally accepted based on the evidence we have, and there should be a proof statement in the form of an analysis document).

This pull request is a proposal to refactor the `extracted` flag to be a property named `researchState`. The data type of the `researchState` property is an enumerated value. Section 4 I renamed to be "Research States" and now includes (in addition to the "extracted conclusion" constraints) the "known research states" enumeration, which is defined as follows:

URI | description
----|-------------
`http://gedcomx.org/Extracted` | The conclusion was extracted from a single source. The conclusion MUST conform to the [Extracted Conclusion Constraints](#extracted-conclusion-constraints).
`http://gedcomx.org/Hypothetical` | The conclusion is a working hypothesis.
`http://gedcomx.org/Accepted` | The conclusion has been formally accepted. The conclusion SHOULD provide a reference to an analysis document that is used to provide the genealogical proof statement.

Comments are welcome.