docs(*): ensure all dist files contain a license header
Changed Gruntfile:
- extended concat task with banner configuration for each lib_file
- Added exclamation mark to the banner to prevent deletion of banner
  during uglify
- added preserveComments option to uglify task to preserve license
  header.
- registered the concat:banner taskts for each lib file.

Partially resolves #428
chore(*): cleanup obsolete grunt config

Since src/service/default-interpolation.js moved from lib_files.ext to
lib_files.core some time ago, some grunt configuration became obsolete.

- Removed obsolete default_interpolation configuration
- Removed obsolete build:default_interpolation grunt task
chore(Gruntfile): ensure all dist files contain a license header
Changed bower.json:
- added licenses array so that the MIT licence is added to the license headers

Partially resolves #428
fix(package.json): fix repository url
fix(demo): correct demo of `translate-values`
feat(service): add possibility to translate a set of translation ids

This enriches both `$translate()` and `$translate.instant()` with the
possibility to call them with an array rather than a string. This makes
the API more comfortable in some situations.

```javascript
// for example...
$translate(['FOO', 'BAR']).then(function(translations){
  console.log(translations.FOO);
  console.log(translations.BAR);
});
// ... and
console.log($translate.instant(['FOO', 'BAR']).FOO);
console.log($translate.instant(['FOO', 'BAR']).BAR);
```
feat(directive): add option to define a default translation text

This adds an optional opt-in feature:

```html
<span translate=„my.key.for.a.welcome.message“ translate-default=„Welcome {{username}}!“></span>
```

or even

```html
<span translate translate-default=„Welcome {{username}}!“>my.key.for.a.welcome.message</span>
```
Update autogenerated documentation on how $translate is used