Support creating, reading, and searching contacts via API.

Summary:
* Update corresponding documentation.
* Remove deprecated functions from old api.py file (keep corresponding unit
  tests as reference for eventually unit-testing the new API functions, but
  mark as xfail).

Test Plan: Manual testing.

Reviewers: mg

Reviewed By: mg

CC: bengotow

Differential Revision: https://review.inboxapp.com/D142