Trading form

Add the buy/sell trading form.

The issuer select disappears if the currency is "STR" or empty.
The total payable amount is calculated from the base amount and unit price.
Creating the offer is handled by the OrderBook.
Fixes #764.

The currency list is populated from the wallet's gateways.
The issuers list is filtered by the currency that is selected.
Part of #762.

![screen shot 2014-09-09 at 3 18 31 pm](https://cloud.githubusercontent.com/assets/3108007/4209928/60efc840-386f-11e4-865e-4e0442cbfc70.png)

What sort of validity are you representing here?  Why do you consider baseAmount and unitPrice? (two things that don't matter to the validity of an order book, but do pertain to the validity of the "make an offer" form)

IMO, the way you've structured your model is a bit wonky and it shows through this method.  Let me see if I can formulate some constructive advice.