janus_flatten_gallery proposal

@biometrics/isi, @biometrics/sri, @biometrics/str & @biometrics/umd:

This proposed patch introduces the following changes:

1. Added `janus_flat_gallery` typedef analogous to `janus_flat_template`.
2. Added `janus_flatten_gallery` function as proposed, offering an interface for gallery learning/indexing/finalization prior to search.
3. Changed `janus_flatten` to `janus_flatten_template` for consistency.
4. Changed `janus_search` and `janus_compare` to operate on `janus_flat_gallery` instead of `janus_gallery`.
5. Bumped version to 0.2.0 to reflect breaking API changes.
6. Misc const correctness and typo fixes noticed while updating the API.

@bklare @sklum @bhklein CC'ing you guys too.