[API] Removed the required `:id` parameter from the "Term Vectors" API

Since Elasticsearch 1.4, the `termvectors` API doesn't require an ID if we pass instead an artificial document.

See: http://www.elasticsearch.org/guide/en/elasticsearch/reference/current/docs-termvectors.html#_example_3_phrase_revisionflag_added_revision_1_4_0_beta1_added_in_1_4_0_beta1_phrase

Closes #115