Add DNT header check support

| Q             | A
| ------------- | ---
| Bug fix?      | [no]
| New feature?  | [yes]
| BC breaks?    | [no]
| Deprecations? | [no]
| Tests pass?   | [yes]
| Fixed tickets | []
| License       | MIT
| Doc PR        | []

This add support for checking the HTTP `HTTP_DNT`-header as send by some browsers.
Added a test for this and manual tested with Firefox 29.0.1, Chrome 35.0.1916.153 m, IE 11.
I would like to add this because even if the DNT proposal gets changed overtime this method can be changed/extented accordingly without having to change applications based on the class.
@see http://en.wikipedia.org/wiki/Do_Not_Track and https://www.mozilla.org/en-US/dnt/
no need to specify the third argument, as it is the default value