Implement new page to select the maturity before checkout

This closes #604.

![screen shot 2014-09-18 at 5 10 10 pm](https://cloud.githubusercontent.com/assets/230476/4329124/760818c6-3f91-11e4-9612-80a5aa416374.png)

![screen shot 2014-09-18 at 5 10 38 pm](https://cloud.githubusercontent.com/assets/230476/4329127/795af3ea-3f91-11e4-858e-698f38b5ea10.png)