Adds fractionable and notional orders

[travishooper]

This change adds to the Asset class a boolean field to determine if an asset can be ordered in a fractionable amount. If the asset is fractionable, a new order can be created for that asset using a float as the quantity. Furthermore, the quantity qty field for a new order is now optional, however new orders require that either the qty or notional argument is passed. Notional allows for a set dollar amount of a fractionable asset to be passed to create a new order.

[travishooper]

@ccjr I was able to record the cassettes today and get the specs passing. This should be good to go now.

[ccjr]

This looks good. I'll merge and release a new version this weekend, thanks!