Allow blank/new lines to contain any kind of whitespace

This fixes a problem where a stray tab on a new line
would cause a parsing error, difficult to debug.
Use govspeak directly instead of depending on a `GovspeakPresenter`

This presenter was only defined in smart-answers
so smartdown would have broken when we attempted to
use it anywhere else.

(TODO: stop using html_safe - do that from smart-answers)
Merge pull request #104 from alphagov/handling_whitespace_simple

Handling whitespace simple