Detect when there is no internet connection and auto-enable offline mode

As per Issue https://github.com/bower/bower/issues/572, `PackageRepository.fetch` does a simple check for Internet connectivity. If it passes, all things proceed as normally. If it fails and neither 'offline' or 'force' are specified, a message is presented and `resolverFactory` is kicked off in 'offline' mode.

If `--force` is used, `--offline` and automatic detection are ignored.

Includes updates to test files.