Implement simple text search index via http - FB 33294

- Create mango_idx_text.erl to handle incoming http request

- Modify mango_native_proc.erl to handle <<"index_doc">>. Function
  get_text_entries returns [[Key1,Val1,[{<<"stored">>,true}]]].
  Dreyfus converts this to [{Key1,Val1,[{<<"stored">>,true}]}].
  Clouseau expects the above format of list of 3 element tuples,
  with the third element being a list of options. Hard-coded now.

- Modify mango_idx.erl and mango_sort.erl to handle new <<"text">>
  index type.