Add identity::assert_equal test (and working implementation)

Add identity::assert_not_equal test (no implementation as of yet)
Add disable_if<T…> for assert_* fallbacks

Modify presentation of API (gearing up for final release)
Modify identity::assert_* functions to take an optional string literal.
Modify run-all test to pass.
Modify self- is-not-null test to pass

Fix typo in amalgamation header (unittest/unittest.hpp)

Remove trait::is_printable<T> (see issue #8 for why)
Remove enable_if<T…> in utility.hpp

This commit gets me closer to a final release for milestone 1.0
Issue #2 is also getting closer to being closed as well, though many
more assertions will be needed.