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)

Correct me if I'm wrong, but when creating an OrderBook it expects 2 currency issuer pairs, and when buying or selling it expects 2 values.