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)

The validOrderBook function in trade-controller deals with currency objects (an order book is identified by a pair).  Notice how the variable names in my version match the parameter names used to create an order book?  IMO that makes it much more understable to the next person to look at the code.

Regardless,  my trade-controller is hacky and a prototype, please don't consider it well factored code.

Regarding validation... this level of validation (whether or not the dropdown has a selection) shouldn't be handled by OrderBook directly.  Instead, if you pass invalid data to the back end, we will throw an error.