Added .gitignore for the DM language.

The DM language is used by the [BYOND](http://www.byond.com/) platform. The files I've added to DM.gitignore are either compiled binary files (`.dmb`, `.rsc`) or files used by BYOND's IDE (either for internal purposes [`.lk`], or user-specific configration files [`.int`]). Lastly, `.zip` files can be generated using the IDE for distributing binaries and other files necessary to run a project, so they have been discluded as well.