Small style adjustments
Update release README
Update docstring for targets, small style fixups
Style clean-up and fix a bad ident impacting logic
Add some readability upgrades to recent classes.
Change Jarsigner to a Task not a Nailgun task

As I found, handling for jars not bootstrapped is a global
issue. That will remain TODO for now, so we lose the Nailgun
and simply subclass Task
Change "type" attribute in Keystore to "build_type"

This matches the attribute in AndroidBinary
Chnage Keystore 'sources' attribute to source
Add BuildTypeMixin to normalize and verify build_type input

Used in AndroidBinary and Keystore tragts. Using a Mxin since they have
no common ancestor in the Android namespace.