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
Bugfix for wrapped elements
Set initial remplate in rendering context
Updated single tags
Ability to clear dev template cache
Throw exception when raiseError is called