Numerapi

- add tournamentId to `get_stakes`
- fixed `stake` after API change on Numerai's side

- support tournament parameter for various endpoints. Numer.ai is planning to
run more than one tournament at a time. This change makes numerapi ready for
that.
- minor code cleanup

- don't query Numerai's API if the action requires an auth token, but there
is none provided
- more & improved tests (test coverage now > 90%)
- consistency threshold moved to 58, following the latest rule change

- use `decimal.Decimal` instead of floats to avoid rounding errors (3)
- optional flag to turn of tqdm's progress bars (4)
- update `check_submission_successful` to recent rule changes (originality
no longer required)
- update documentation

- import NumerAPI class to toplevel. now `from numerapi import NumerAPI` works
- added `get_dataset_url`
- more & improved tests

- added `check_new_round` to check if a new round has started
- added `check_submission_successful` to check if the last submission passes
concordance, originality and consistency
- return proper Python data types, for example the NMR amounts are now
floats and no longer strings
- show progress bar while downloading dataset
- general code cleanup & more tests