Numerapi

- removed `filename` from `get_user_activities`, no longer supported.
- rename `get_submission_filename` to `get_submission_filenames`
- `get_submission_filenames` now only works for the authorized user. It allows
to get ones submission filenames, optionally filtered by round_num and
tournament.

- added `get_rankings`, which gives access to numerai's global leaderboard
- added `get_user_activities`, that allows to see each user's submission and
staking activity
- added `get_submission_filename` to get the submission filename for any user,
tournament & round number combination
- added `prizePoolNmr`, `prizePoolUsd` and number of `participants` to the
`get_competitions` endpoint
- ensure functionality of command line interface is in sync

- added `get_tournaments`
- added `tournament_name2number` and `tournament_number2name` to translate
between tournament numbers and names

- added numerapi command line interface
- allow passing public ID and secret key via environment variables

- added `stakeResolution` information to get_leaderboard
- added badge for read the docs to README

- publish README as long_description on pypi
- fixed `get_transactions` after API change on Numerai's side
- added proper docstrings to all public methods, using Google Style as
described at http://sphinxcontrib-napoleon.readthedocs.io/en/latest/example_google.html
- added examples for all public methods
- added documentation on readthedocs: http://numerapi.readthedocs.io