Add missing property `namespace` to `DS.RESTAdapter`

It is kindness that public API is clearly specified.
Move unit test for JsonSerializer to serializers directory

The test for the RESTSerializer already is in this directory so this
change adds some consistency.
`DS.Adapter.resolveMapConflict` can receive two arguments
Better YUIDocs
Eliminate unused variables

I used jshint with '"unused": true' option.
bump ember to rc2
Ember source now comes from the ruby gem.
link to s3 builds in README