Added the ability to set a symbol instead of method call for styling

Added the ability to set a symbol which correlates to a method call to pull in styles.

```ruby
# in the styles module
def my_view_styles
  {
    background_color: UIColor.redColor
  }
end

# old way (still works)
def on_load
  add UIView.new, my_view_styles
end

# new way
def on_load
  add UIView.new, :my_view_styles
end
```

This is a purely stylistic/cosmetic change, but it makes the API feel more "CSS-y" which I feel is a win. I'll be modifying the docs to favor this approach.

Added a test for this as well.

```
# rake spec:unit
202 specifications (330 requirements), 0 failures, 0 errors
```