Adding postinstall script

Now, all you need to do is run `npm install` and you'll have everything you need for the tests to pass. Previously, if you ran `npm install && npm test`, the tests would fail because the bower components were not available. This is the error you would see:

```
$ npm test

> angular-translate@1.1.1 test /Users/nick.heiner/public-projects/angular-translate
> ./node_modules/karma/bin/karma start karma.unit.conf.js --browsers Firefox --single-run

[2014-02-14 15:15:12.879] [DEBUG] config - autoWatch set to false, because of singleRun
INFO [karma]: Karma server started at http://localhost:9876/
INFO [launcher]: Starting browser Firefox
WARN [watcher]: Pattern "/Users/nick.heiner/public-projects/angular-translate/bower_components/messageformat/messageformat.js" does not match any file.
WARN [watcher]: Pattern "/Users/nick.heiner/public-projects/angular-translate/bower_components/angular/angular.js" does not match any file.
WARN [watcher]: Pattern "/Users/nick.heiner/public-projects/angular-translate/bower_components/angular-cookies/angular-cookies.js" does not match any file.
WARN [watcher]: Pattern "/Users/nick.heiner/public-projects/angular-translate/bower_components/angular-mocks/angular-mocks.js" does not match any file.
INFO [Firefox 24.0 (Mac)]: Connected on socket id IF12VzBM2VQpr-jDCeQB
Firefox 24.0 (Mac) ERROR
	ReferenceError: angular is not defined
	at /Users/nick.heiner/public-projects/angular-translate/src/translate.js:8
Firefox 24.0 (Mac): Executed 0 of 0 ERROR (0.049 secs / 0 secs)
npm ERR! Test failed.  See above for more details.
npm ERR! not ok code 0
```