Add `app.import` to control legacy files, remove broccoli-bower.

### Summary

This PR removes `broccoli-bower` since the `bower.json` spec doesn't provide enough information on exactly what should and shouldn't be imported, and it adds a new `EmberApp.prototype.import` method.  `EmberApp.prototype.import` is used both internally and in your Brocfile to manage your dependencies.

### Usage

In your `Brocfile.js` specify a dependency  before calling `app.toTree()`. The following example scenarios should illustrate how this works.

#### Javascript Assets

##### Standard Non-AMD Asset

Provide the asset path as the first and only argument:

```javascript
app.import('vendor/momentjs/moment.js');
```

##### Standard AMD Asset

Provide the asset path as the first argument, and the list of modules and exports as the second:

```javascript
app.import('vendor/ic-ajax/dist/named-amd/main.js', {
  'ic-ajax': [
    'default',
    'defineFixture',
    'lookupFixture',
    'raw',
    'request',
  ]
});
```

##### Environment Specific Assets

If you need to use different assets in different environments, specify an object as the first parameter. That objects keys should be the environment name, and the values should be the asset to use in that environment.

```javascript
app.import({
  development: 'vendor/ember/ember.js',
  production:  'vendor/ember/ember.prod.js'
});
```

##### Customizing a built-in Asset

This is somewhat non-standard, but suppose that you have different versions of Ember specified (using the canary builds for example).  You would simply manipulate the vendor tree that is passed in to the `EmberApp` constructor:

```javascript
var EmberApp = require('ember-cli/lib/broccoli/ember-app');
var fileMover   = require('broccoli-file-mover');

var vendorTree = fileMover('vendor', {
  files: {
    'ember-dev/ember.js': 'ember/ember.js',
    'ember-prod/ember.prod.js': 'ember/ember.prod.js'
  }
});

var app = new EmberApp({
  name: require('./package.json').name,
  trees: {
    vendor: vendorTree
  }

  getEnvJSON: require('./config/environment')
});
```

#### Styles

##### Static CSS

Provide the asset path as the first argument:

```javascript
app.import('vendor/foundation/css/foundation.css')
```

All style assets added this way will be concatenated and output as `/assets/vendor.css`.

##### Dynamic Styles (SCSS, LESS, etc)

The vendor trees that are provided upon instantiation are available to your dynamic style files.  Take the following example (in `app/styles/app.scss`):

```scss
@import "vendor/foundation/scss/normalize.scss";
```