Symbol spike

this is just a spike, experimenting with symbols over defProp

- [ ] make this work with non-symbol supporting browsers
- [ ] evaluate the impact of slower property lookups but much faster object allocation.. (until they make symbol lookups faster)
- [ ] fix jshint
- [ ] test both paths.

this reduces the cost of ember object allocations by 4x-6x and seems to improve perf-stress render time by 10% or so.