Extract normative assertions for testing

[mmccool]

For defining test cases we need to extract and name all the normative assertions.

I have automated this process but it requires following some conventions in the markup of normative assertions. I'll test the automation first in the TD spec and when it's reasonably stable can make a similar PR for the scripting API spec.

[zolkis]

I would argue that it's enough to use MAY, MUST, SHOULD, as ReSpec will handle them as normative assertions and every occurrence is as well defined as it would be by a special tag. There is no separate normative and non-normative use of MAY, MUST, SHOULD, etc. So let's keep the specs clean.

[danielpeintner]

Close in favor of #190