ProGuard/R8 rules

[EddieRingle]

Adding R8 to our application, it seems there's some reflection being done in Squeaky, so the following rule was necessary to retain what was being accessed:

-keep class co.jasonwyatt.squeaky.DatabaseHelper { *; }

It'd be a handy improvement if rules could be bundled with the library.

[jasonwyatt]

Thanks! Ultimately I'd like to remove the reflection all-together and use annotation processing. Hopefully I'll have some time to devote to that in the next few months.