Row-level (or per instance) language specification for FTS

This patch allows to use some field in base model to be considered by watson for creating ts_vector. This gives ability to use different languages for different instances of base model.
For example:

    class MyModel(models.Model):
        search_language = models.CharField(_("Language for search index"),
                                                              max_length=24,
                                                              default="russian")
       content = models.TextField()

    watson.register(MyModel, search_config='search_language')

then we can use watson.search() and filter() with search_config paramer for particuar searching, e.g. in some view:

    if request.LANGUAGE_CODE == 'ru':
        res = watson.search(query, models=(model,), search_config='russian')
    else:
        res = watson.search(query, models=(model,))  # for default search config

NOTE: There isn't any tests yet, but code was checked on sample data.
Please review this code