Remove invisible system files from plugin root

There was/is a hidden `.DS_Store` file in the root of your plugin. Mac OS X uses this file for storing custom folder properties. There's generally no need to commit these files with projects, or even for your Git project to be aware of them.

This commit removes the `.DS_Store` and introduces ` .gitignore` file, which contains a list of possible files names that the current Git project should ignore (i.e. not listen to changes for). This will help prevent these files creeping back in the future.