Use` _language` field instead of `language`

 When we want to force a language instead of using Tika language detection, we set `language` field in documents.

 To be consistent with other forced fields, `_content_type` and `_name`, we should prefix `language` field by an underscore `_`.

 So `language` become `_language`.

 We first deprecate `language` in version 2.1.0 and we remove it in 2.3.0.

 Closes #68.