DSL: document the new-style header line

Big change!  The header line adopts this form:

```ruby
cask :v1 => 'google-chrome' do
```

We can forget about the complicated classname conversion rules; the header name maps to the filename.  Just remove the file extension, and place it in quotes.

The classname is still there behind the scenes, so something may trip Travis now and then while we remove backward-compatible code.  Limitations such as being unable to start the Cask with a numeral are still present, and documented, but in the process of being removed.

There was a fairly large oversight: the test suite can't be converted to the new form yet, because test Casks use a special class `TestCask`.

This PR also covers devscripts and the `brew cask create` template.

Refs: #5365