Add flycheck-dmd-include-path variable [#343]

The new variable passes in the include directories to dmd with the "-I" flag.
Refactor D tests

Move all D source files into a sub-directory, to create a package
hierarchy.
Add test for missing D import
Remove superfluous D test cases
Wrap overlong line
Update changelog and credits
[Fix #344] Merge branch 'atilaneves-d-dmd-includes'

* atilaneves-d-dmd-includes:
  Update changelog and credits
  Wrap overlong line
  Remove superfluous D test cases
  Add test for missing D import
  Refactor D tests
  Add flycheck-dmd-include-path variable [#343]
Rebuild Texinfo manual