Missing feature: add filters

[tbosch]

Filters should be defined just like directives via a class that has the @Filter annotation, e.g. @Filter({name:'multiply'}).

In a template, only those filters should be allowed in expressions that are provided via the <module> tag, just like directives (resp. when calling compile manually the allowed filter classes need to be given as an argument to compile).

Open questions:

• when a directive calls view.watch(expr...), which filters can this expression contain? Would the directive need to provide the filters as an argument to watch?
• on which injector are filters registered? On the root injector or on the view's injector?