Finalize ExpressionAssertion

[weierophinney]

ExpressionAssertion was proposed in #23. This patch does the following:

• Rebases it from develop.
• Adds operators for strict in/equality and negated regex.
• Renames the "not in" operator to !in.
• Removes a potential concurrency issue.
• Refactors internals for readability, maintainability, and context.
• Adds documentation.

[nikolaposa]

Thanks @weierophinney ! Sorry for locked branch complications, not sure how/why it happened.