generating mainDexClasses when multiDex flag set to true and mainDexList...

... is null.

This is based on http://blog.osom.info/2014/10/generating-main-dex-list-file.html

Uses mainDexClasses scripts found in newest build-tools 21.1.1. It won't work without it.

This is a work in progress, I haven't checked it on linux box yet. I will write some integration test for this as well in my spare time.

Maybe this should be in a separate mojo, surely the filename could be configurable. Or maybe there should be a parameter such as dexGenerateMultiDex?