Support for Apple Watch

[cparnot]

I started using ASCIIImage with some Apple Watch code and noticed that the automatic scaling done by the current code will be done based on the current device scale, which can be wrong if running in the watch extension. For instance, if running on iPhone 6+, you'll get a 3x image, when you actually need 2x.

There are methods normally exposed only for tests that have a scaleFactor parameter that can be used as a workaround, but the naming can be confusing and there is the issue #1 that needs to be taking into account.