Add option to supress 'open' call

This is useful when automating, you dont always need a finder window
right away...

Example:

BRIAR_DONT_OPEN_ON_RESIGN=1 briar resign app.ipa ...

- Will not break old behavior
I try to follow the calabash-ios convention for environment variables:

```
if ENV['BRIAR_DONT_OPEN_ON_RESIGN'] == '1'
```

http://calabashapi.xamarin.com/ios/file.ENVIRONMENT_VARIABLES.html#Conventions

This change is fine as it is, but you might find the behavior changes in future versions.