[BUGFIX beta] Use attrMorph, properties before attributes

HTMLBars is responsible for managing escaped and sanitizaition of bound
attributes. It uses an AttrMorph construct to do this. Consequently the
bind-attr and htmlbars attr syntaxes are much simpler to implement.

Conflicts:
	packages/ember-htmlbars/lib/helpers/if_unless.js
	packages/ember-htmlbars/lib/helpers/unbound.js
	tests/node/template-compiler-test.js