Name Relationship helper: toMany and toOne

[pixelhandler]

Change relationship helper names: toMany and toOne instead of hasMany and hasOne

See nomenclature here:

• http://jsonapi.org/format/#crud-updating-to-many-relationships
• http://jsonapi.org/format/#crud-updating-to-one-relationships

We could keep the hasMany and hasOne as aliases as well. Here is a branch I started to explore the change https://github.com/pixelhandler/ember-jsonapi-resources/compare/attr-helpers-naming