Basic working equiv input type, with instant validation.

doc/en/Authoring/Inputs.md

doc/en/CAS/Equivalence_reasoning.md

doc/en/Developer/Development_track.md

question.php

stack/input/ajax.php

stack/input/equiv/equiv.class.php

stack/input/inputbase.class.php

stack/input/matrix/matrix.class.php

stack/input/textarea/textarea.class.php

stack/maxima/stackmaxima.mac

tests/input_textarea_test.php

version.php

yui/input/input.js