Add support for different production / development environments

This __isn't__ ready to be merged in -- but would this be a helpful addition? I can spend some more time on it if so.

Basically, I have multiple 'production' environments. One for regular website, one for Phonegap.

Currently, running 
```bash
ember build --environment=phonegap
```
Raises an error (0.0.27/master -- didn't cause error in 0.0.25) in EmberApp.prototype.import for not matching Ember development / production version in EmberApp.prototype.populateLegacyFiles.

This patch lets me add this to my Brocfile.js

```javascript
var app = new EmberApp({
  name: require('./package.json').name,
  getEnvJSON: require('./config/environment'),
  addProductionEnvironments: ['staging'],
  addDevelopmentEnvironments: ['demo']
});
```

Where I can then build
```bash
ember build --environment=staging
```
which inherits from production defaults, and allows me to manage custom environment differences in config/environment.js