Various fixes

(This is in connection with https://github.com/milkypostman/melpa/pull/1590)

- Shorten package description line
- Fix documentation errors reported by checkdoc
- require flycheck, since we refer to `flycheck-dmd-include-path`
- Add missing spaces before function argument lists
- Provide an autoloaded setup function, and tell the user to add it to `d-mode-hook`

Further feedback: 

- I'd suggest using `expand-file-name` rather than `concat` when building file paths. I didn't make that change because I can't easily test it.

-Steve