Wrap basic threads retrieval and traversal functionality

Namely:

    -   notmuch_query_search_threads()
    -   notmuch_threads_valid()
    -   notmuch_threads_get()
    -   notmuch_threads_move_to_next()
    -   notmuch_threads_destroy()
    -   notmuch_thread_destroy()
Wrap notmuch_query_set_sort() and notmuch_query_get_sort()
Wrap notmuch_thread_get_thread_id()
Wrap notmuch_thread_get_subject()
Document the structure of the test corpus
Wrap notmuch_thread_get_total_messages()
Wrap notmuch_thread_get_matched_messages()
Wrap notmuch_thread_get_authors()