Add .gitignore and remove per-user state files

[apjanke]

I suggest adding a .gitignore like this, and not keeping per-user files under source control.

When I was working on #58, I got some spurious diffs, since I was doing the build as a different user. I think you'll see more of those if you collaborate with other people on this project. This change will eliminate those spurious diffs.

$ git status
On branch master
Your branch is up-to-date with 'origin/master'.
Untracked files:
  (use "git add <file>..." to include in what will be committed)

    class-dump.xcodeproj/project.xcworkspace/xcshareddata/
    class-dump.xcodeproj/project.xcworkspace/xcuserdata/janke.xcuserdatad/
    class-dump.xcodeproj/xcuserdata/janke.xcuserdatad/

nothing added to commit but untracked files present (use "git add" to track)

Based on Xcode-specific gitignore from http://stackoverflow.com/questions/49478/git-ignore-file-for-xcode-projects