Add EmberCPM.Macros.allEqual.

@linstula / @rjackson
The [`equal` macro](https://github.com/emberjs/ember.js/blob/54dad24457659e93973be55faad15ffc78f2433f/packages/ember-metal/lib/computed_macros.js#L229-L258) takes a property name and a reference value.

That makes this a bit inconsistent with that. One idea would be to change this to `allIdentical` to avoid the similarity in naming.

Regardless, it would be helpful to specify in this documentation that they are all equal *to one another* rather than some reference value.