Implement Theming

This is an unsquashed series of commits that implement theming in the app.

Selecting a theme, for a user, is as simple as the following:

```<md-button md-theme="green" class="md-primary">Say Hello!</md-button>```

Notice that we have a new `md-primary` class that gets used to apply the theme's primary color to things. It is currently used in `md-button` and `md-subheader` (for a colored subheader).

This also converts button's classes to `md-raised` and `md-fab` instead of `md-button-raised` and `md-button-fab` because we are already namespacing with the `md-` prefix, so there shouldn't be any collision. Ultimately this could be used for other "raised" things, such as a "raised" control.

To stay consistent, toast now has a `md-action` class for highlighting things. In the demo specs, it uses yellow to highlight, so notice that `md-action` is not `md-primary` as it does not use the theme's primary color.

This also moves a component's variables into its `_componentName.scss` file, which simplifies managing the components.

Also, I enhanced the demos of button (add a `md-warn` button), and dialog (opening a simple dialog that isn't long).



We can't add a watch for every component, especially one that watches a string. This running every digest could quickly get expensive.  

I think we should *not* watch the theme by default, and add an additional directive, maybe `md-theme-bind` that does this.