Export strict assertion mode functions

[strugee]

Apparently Node core now has a strict assertion mode, and the old behavior ("legacy assertion mode") is deprecated. See https://nodejs.org/api/assert.html#assert_strict_assertion_mode.

I think we should probably make require('vows').assert export strict mode functions, since the legacy mode functions a) are deprecated and b) cause surprising results since they use ==. Assuming we do this, we should do it before 1.0.0 final because it's a breaking change.