Add global .gitignore file

[blakek]

A good, default, global .gitignore file should be added.

A starting place:

# Silly Apple, Git is for developers!
.DS_Store
.DS_Store?
.AppleDouble
.LSOverride

# Thumbnails
._*

# Files that might appear in the root of a volume
.DocumentRevisions-V100
.fseventsd
.Spotlight-V100
.TemporaryItems
.Trashes
.VolumeIcon.icns
.com.apple.timemachine.donotpresent

# Directories potentially created on remote AFP share
.AppleDB
.AppleDesktop
Network Trash Folder
Temporary Items
.apdisk