Move the default realm path on OS X to Application Support

And when not running in a sandbox, use the bundle identifier as a
subdirectory name so that each application has its own default realm
file rather than sharing them, falling back to the executable name if
it's not running in a bundle.
Create the directory containing the realm file if it does not exist
Merge pull request #719 from realm/tg-osx-realm-path

Move the default realm path on OS X to Application Support