Update .gitignore to upstream version.

Updates from the default .gitignore to the latest Objective-C.gitignore
from github/gitignore@0aeefb48c0ac885a516fb92f7f5bfd7e85ce7a37.

Among other changes, this allows .xcworkspace files to be committed.
New workspace.
Merge branch 'initial-workspace'

Create an initial workspace to hold JBAtem files during development.