Trading form modal statuses

Show a summary of the offer.
Disable the submit button until the form is valid.
Confirm the trade before submitting.
Show a sending modal status. (TODO: Use gif with grey background)
Show a sent modal status.
Show an error modal status.

Fixes #819.

![screen shot 2014-09-18 at 11 55 46 am](https://cloud.githubusercontent.com/assets/3108007/4324773/cfbc6518-3f5d-11e4-94b9-0c7e12d46e6f.png)

![screen shot 2014-09-18 at 11 56 10 am](https://cloud.githubusercontent.com/assets/3108007/4324775/cfbf9878-3f5d-11e4-8557-d70536b3fc2e.png)

![screen shot 2014-09-18 at 11 56 30 am](https://cloud.githubusercontent.com/assets/3108007/4324772/cfbb55c4-3f5d-11e4-83b5-f77212485bad.png)

![screen shot 2014-09-18 at 12 00 47 pm](https://cloud.githubusercontent.com/assets/3108007/4324776/cff4b170-3f5d-11e4-8b80-85b0397c2c12.png)

![screen shot 2014-09-18 at 11 58 35 am](https://cloud.githubusercontent.com/assets/3108007/4324774/cfbd9f50-3f5d-11e4-92fe-4cf2d439aae4.png)

![screen shot 2014-09-18 at 12 02 45 pm](https://cloud.githubusercontent.com/assets/3108007/4324807/2426d30e-3f5e-11e4-8da1-8470ea0fd024.png)