Available helper methods in custom views

Available helper methods panel, columns, status_tag and blank_slate in custom views.

Example:
```haml
-# /app/views/admin/profiles/compare.html.haml
= columns do
  = column do
    = panel '#1' do
      = status_tag('active', :error, label: 'red')
  = column do
    = panel '#2' do
      = status_tag('active', :ok, label: 'green')
  = column do
    = panel '#3' do
      = status_tag('active', :yes, label: 'blue')

= blank_slate 'Blank slate'
```

Result:
![custom_view](https://cloud.githubusercontent.com/assets/700998/4175113/97386f54-35bf-11e4-8ce1-f99df00857ea.png)