Ember can v2

[Exelord]

This PR will prepare for v2.0 release

Issue: #82 #89

New features

1. If you pass undefined as a model to can or cannot it do not override a model defined in class definition.
2. You can override property parser per each Ability class, overriding parseProperty() method to use eg. custom prefix like is (isReadable) instead of can (canReadable)
3. You can now get a value for specific ability using CanService#valueFor, eg. this.can.valueFor('createProjects', 'account', model, properties)

Deprecations

1. You will not longer be able to import CanMixin as it will be removed. You can reproduce its behavior using can service and its methods.
2. import { CanService } from 'ember-can' -> import CanService from 'ember-can/services/can'
3. import { ability } from 'ember-can' -> import { ability } from 'ember-can/computed'
4. build method of can service will be replaced by abilityFor

Breaking changes

1. You have to pass a null model if you want to pass only parameters to can or cannot methods of can service. Eg: this.get('canService').can('post', null, { test: true })
2. The model passed to helpers or can service methods, as from now on, can be anything instead of an Ember.Object instance.
3. can and cannot methods will always return a boolean
4. Abilities are now an instance of the Ability class, so setting model or properties on it will not modify the class constructor anymore.
5. Ability class name will be always singularized automatically, eg. edit posts -> edit post. All abilities files should be created with the SINGULAR form
6. Drop support for Node 6
7. Drop support for ember < 3.4

[villander]

LGTM!