Significant refactor of module

Adds selective TFA based on "protected" admin ACLs set by config.xml and editable in System>Config, also adds Composer support. Tested and working in production. Also adds framework for supporting TFA in frontend (incomplete/untested).

As started by @aedmonds @ramblindan @willtran and @svenvarkel at Imagine Hackathon, subsequent contributions from @andrew-dwyer. ping @philwinkle