mg update to 20130922

This PR consists of two parts: the mg update and a new dependency for mg called clens.

    Update mg to 20130922
    This brings in two years of fixes and features for mg.

and

    Add formula for clens 0.7.0
    clens is a convenience library to aid in porting code from OpenBSD
    to different operating systems. The patch that's refenced in this
    formula has already been applied upstream, but no new release has
    been made yet.