Add `EmberCPM.Macros.join`

`EmberCPM.Macros.join` joins the result of each dependent property into a
single string, with the supplied separator.  This is analogous to
JavaScript's Array.join, which is used inside the implementation.

Example:

```javascript
person = Ember.Object.create({
  firstName: 'Jean-Luc',
  lastName:  'Picard',
  fullName:  Ember.computed.join('firstName', 'lastName', ' ')
});
```

Note, this is very similar to
https://github.com/emberjs/ember.js/pull/3032 however I have moved
the separator argument to the end to be more consistent with EmberCPM.
Add `join` to README.
fmt macro supports multiple keys

For backwards compatibility, it will return `undefined` if *any* of the
input values is `undefined` and `null` if *any* of the input values is
`null`. Otherwise, this was modeled after
https://github.com/ebryn/bugzilla-ember/blob/master/app/utils/fmt.js

Closes #3
add changelog w/ versions 1.0.0, 1.0.1
Register Ember-CPM version with Ember.
v1.1.0

Feature: Add `join` macro

Feature: `fmt` macro accepts multiple properties; the `formatString` is
still the last argument, so this is 100% backwards-compatible with the
single-property version.

Register EmberCPM with `Ember.libraries` if available.