DSL: document the new-style header line

[rolandwalker]

Big change! The header line adopts this form:

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

[tapeinosyne]

Post hoc +1. Despite the absence of architectural changes, it is desirable to update our headers, as human-friendlier names simplify contributions.

[rolandwalker]

Architectural changes are intended to follow as well, though the relevant conversation is scattered.

• The backend can be refactored to use instances instead of classes, improving clarity and separation of concerns.
• Naming limitations can be simplified to the limitations imposed by the filesystem.
• Casks can be read in as a single do block, the evaluation of which can be deferred. This should bring a general performance benefit, and protect against individual slow Casks as bit us in the past. Performance is not crucial here but we do need room to grow.