links#find should now be resolved with store.findLinked

Source#findLink now fetches IDs, while findLinked fetches records.
Moved attr, hasMany, and hasOne to fields dir

This is in preparation of introducing `key`.
Compatible with new flexible key schemas in Orbit

* There is no longer a single `idField` and `remoteIdField` per schema
* Every model can have a unique primary key and any number of secondary keys
* Keys are defined with the new `key` helper
* The primary key value is always mapped to `primaryId` on each model
* If no primary key is defined for a model, a UUID-based `id` field will be defined automatically
Fix Model.getKey
orbit 0.5.0
Describe keys and schema customizations in README
Allow keys to be defined without specifying a type

The default type `string` will be used by default.
Convert Model#primaryKey to a proper CP
Update to the export file.
Implement missing findLinked used in LinkProxyMixin.
Merge pull request #30 from ProlificLab/local_master_2

Implement missing findLinked used in LinkProxyMixin.
Initialize new properties to {} in Store#add

This allows a new object to be added with no data.
Reorder
Merge pull request #29 from ProlificLab/local_master_1

Update to the export file.
Makes it possible to remove a "hasone" link by setting it to null or undefined.
orbit 0.5.1
0.3.0
Merge branch 'local_master_3' of https://github.com/ProlificLab/ember-orbit into pull-32
Fix + test adding + removing hasOne relationships
Schema#normalize now normalizes links

Thanks to @gnarf for his work on this.

[Fixes #31]