v-repeat optimization

- When the array is swapped, will reuse existing VM for any
  element present in the old array. This greatly improves
  performance when the repeated VMs have a complicated
  nested structure themselves.
- Fixed issue when array is swapped new length is not emitted
- Fixed v-if not removing its reference comment node when unbound
- Fixed transition when element is invisible the transitionend
  callback is never fired resulting in element stuck in DOM
v-repeat object first pass, value -> $value
fix v-repeat reuse vm insertion when detached by v-if
repeat object sync and tests
skip transition for reused nodes
sync back inline input value if initial data is undefined
vm.$set no longer need to check owner VM at run time
subclasses can now use asset registration methods too
{{>yield}}
v-data
__observer__ => __emitter__ for better clarity