added support for "*" parameter in remove methods

This commit makes it possible to remove all translations of a given key, by passing a * instead of a language identifier. This works in both `removeTranslations()` and `removeLanguage()`. Fixes #3.

Examples:

```javascript
// Remove all translations of the title property 
Collection.removeLanguage(id, ['title'], '*')
Collection.removeTranslations(id, ['*.title'])

// Remove all translations
Collection.removeTranslations(id, ['*'])
```