Add user controller with #show and #new (signup)

Resolves #11. Add a basic form that allows user creation. No sessions yet.
How about id's of each flash symbol? Here what I was thinking:

```
- flash.each do |key, value|
      = content_tag(:div, id: key) do
        = content_tag(:span, value, class: "alert alert-#{key}")
```
Seems a little less fragile.