Closes#351
When Sparkle updates Atom, it deletes the app and replaces it. This
causes the `atom` cli to fail when called within this brief time
period. This gives `atom` a five second grace period if it is not
found.
Previously this was done during `rake install`.
Also default to `~/github/atom` as the default resource path
when no `--resource-path` argument is specified. This argument
will now be required when running in dev mode if the repository
is not at the default location.
Closes#300
