Support the translate-namespace directive

Namespaces are supported by grunt-angular-translate, but not the https://angular-translate.github.io/docs/#/api/pascalprecht.translate.directive:translateNamespace directive. The `translate-namespace` directive allows a developer to avoid repeating the namespace in a view by extracting it to a parent element.

That is, given the following example:

```

Expected:

```
{
"CONTENT": {
"HEADERS": {
"TITLE": "",
"WELCOME": ""
}
}
"COMMON": {
"FIRST_LINE": ""
}
}
```

Actual:

```
{
"0": {
"TITLE": "",
"WELCOME": ""
}
"COMMON": {
"FIRST_LINE": ""
}
}
```