DOC: Added contributors to README.md
ENH: Extended commission PerShare method to allow a minimum cost per trade.

Also added unit tests (test_perf_tracking.TestCommissionEvents) to test
all commission models.
ENH: Provide all drawdowns and max drawdowns in cumulative risk.

The values are not part of the risk report, but can be useful for
examining the behavior of the drawdown calculations.
MAINT: Remove unused rounding method.

round_to_nearest is no longer referred elsewhere.
BUG: Fix max drawdown calculation.

The input into max drawdown was incorrect, causing the bad results.
i.e. the `compounded_log_returns` were not values representative of
the algorithms total return at a given time, though
`calculate_max_drawdown` was treating the values as if they were.
Instead, use the `algorithm_period_returns` series, which does provide
the total return.

Update risk answer key with an Excel calculation of max drawdown
to help corroborate the calculations.

Also, remove `compounded_log_returns`, (which actually had stopped
being the `compounded_log_returns` at some point), since the max
drawdown was the only calculation using the values in that series.