Add link to terms of service on create account form.

Meant to resolve https://github.com/hypothesis/vision/issues/94. Here's what it looks like:

![hypothesis](https://cloud.githubusercontent.com/assets/521978/4776217/a285496a-5bbf-11e4-9a6a-0ea466826829.png)

Maybe an open question:
* Do we want a checkbox next to the terms of service?