ENH: Restrict limit prices to a penny precision

to account for minimum price variation.

On an order to buy, between .05 below to .95 above a penny, use that penny.
On an order to sell, between .05 above to .95 below a penny, use that penny.
MAINT: Ensure the sign of the result is positive
Merge pull request #220 from quantopian/minimum_price_variation

Merging branch "Minimum price variation"

Ensure limit prices conform to a minimum price variation of a penny, which works for most stocks (not, for instance, BRK). The rounding "midpoint" is custom and depends on buy/sell direction, instead of .5 of a penny.