Correct xtf::evaluator::handle_* parameter names
Add the xtf::failure enumeration

The xtf::failure enumeration represents the possible levels of failure
severities produced by false statements.
Update xtf::certainty entry documentation

This change is to reflect the introduction of the xtf::failure type.
Add xtf::to_lower function
Add xtf::certainty input formatter
Add xtf::statement input formatters

Change primary xtf::statement output formatter to write identifiers
Add xtf::failure input formatter

Change xtf::failure output formatter to write identifiers