docs(ng-class): expound on the map syntax

Closes #221
refactor(parser): pass FilterMap to expression eval instead of parser constructor
test(ng_mustache): added a test for filters
fix(parser): disallow filters in a chain and inside expressions
fix(selector): Allow two directives with the same selector

Fixes #471
Closes #481
style(forms): change currentErrors to errors
chore(pubspec): move unittest into dev_dep
chore(demo): Add simple controller to TodoApp
chore(test): add missing test for ng-change

Closes# 399