On the path of allowing for tests to easily set up data across multiple
countries.
Most of the existing overrides have just been updated to take the new
signature without any other changes. They'll be update to incorporate
the new parameters in a subsequent commit.
* BUG per unit commissions should not be negative when shorting or selling
* BUG treat commission amounts as positive in Position object even if short position
* MAINT update example data for changes in short orders
* MAINT update example data for changes in short orders under python 3.5.2
* REV keep cost_basis consistent to what it already was
* STY removing unneeded line break
* TST add coverage for commissions' impact on cost_basis for shorts
