Close #187. Add Garden Model, simplify Sign Up flow

# What's changed.
* Gave each user a has_many gardens relationship.
* Moved a number of "gardeny" things out of the user model and into the garden model.
* Removed several fields from sign up form, designed sign up form.

![screenshot 2014-10-15 12 05 17](https://cloud.githubusercontent.com/assets/485583/4640759/e5530730-5428-11e4-9f8e-a6a2a11425d6.png)

# What's next
* A page for adding details (will probably involve Angular)
* A page for adding a garden (will probably also involve Angular)
Are you sure about removing `password_confirmation` from the whitelist attrs? How will users change their password after they sign up? Going to investigate this one this morning to make sure.