Tensile plasticity 3626

 @tonkmr , @amjokisaari , @chandanaj , @cpritam , @rpodgorney , @huangh-inl , one of you might like to read this.  I have documented the background theory at

http://mooseframework.org/wiki/PhysicsModules/TensorMechanics/Tensile/

but i have not yet documented the test suite.  Hopefully in the coming days i'll get time to do that, as well as draw some pictures.