Allow partial emails when searching for orders

Using the `email_field` tag makes it impossible to search for orders by a partial email address using Chrome or Firefox:
![screen shot 2014-08-26 at 5 59 29 pm](https://cloud.githubusercontent.com/assets/540762/4138722/c94c3f1c-3393-11e4-858e-13ab7e581af9.png)