Template improvements

MixinEncounter and FragmentEncounter now share a common interface
TemplateEncounter. A new namespace dust:setters introduced to allow some
limited setting of values from templates.
Resolve methods even if not exact type match
Locations now support resolving new locations
withResource for ResourceVariantResolution
Removed dust-css-sprites

Not really maintaned.
Introduced AssetPipeline
Actually only look for setters in components
Documentation fixes