Added operator prefixes, escaping, and sql query translation

Operator prefixes ("!" for boolean inversion, and "-" for numeric negation) implemented.

Backslashes for escaping special characters implemented.

Escaped parameters implemented (so that parameters with minus signs or spaces in their names can be used).

Also implemented the ability to translate a valid govaluate expression into a SQL "where" clause, as a string. Support for MongoDB queries is experimental.

Contract-compatible with prior releases; expressions and code written for previous versions will operate exactly the same with this release.