Added initial D language support.

Added initial [D language](http://dlang.org/) support.

This is based on quite an old patch that's been floating around for a while. Even though it is old and could be improved, it does indeed work.

This is a recent discussion on the D newsgroup regarding ctags support:
http://forum.dlang.org/thread/klcykihcmttwqqupzgpg@forum.dlang.org if this pull request is accepted i'll direct them here instead of maintaining yet another repo.

Perhaps this should have additions for the other 3 `DECL_` items added:
https://github.com/fishman/ctags/pull/133/files#diff-9c07e5e602aac928d6d779592925a132R155

I don't think it's currently used though, but if we're modifying it anyway...