Added ability to set _all mapping field

Documentation:
http://www.elasticsearch.org/guide/en/elasticsearch/reference/current/mapping-all-field.html

Use case:
```
static searchable = {
  all = [analyzer: 'russian_morphology']
}
```

```
static searchable = {
  all = false
}
```

```
static searchable = {
  all = [enabled: 'false', term_vector: 'with_positions_offsets']
}
```