Add D programming language support

This pull request adds support for D programming language (http://dlang.org/).

It adds a syntax checker for D using `dmd` which is a reference compiler for D (http://dlang.org/download.html).

It also adds a unittesting checker for D using `rdmd` which is bundled with `dmd`.
D has a unit test feature as a language specification and we can easily run the unit tests by using `rdmd` without explicit compilation step.
So I think it is worth adding unit testing checker to flycheck.