chore(Hop build process)

This is a research project to see if the Hop library (https://github.com/dart-lang/hop) would be a good candidate to automate the build process in a platform agnostic way.  This was inspired by @bgourlie and issue #482. I implemented just enough of the build process to get a conversation going, the unit testing is not implemented, but from what I've seen so far I'm pretty confident it can be done.  An alternate approach to this would be to use Gulp, which is what I'm going to experiment with next.

Please take a few minutes to check it out and share your opinions.  Thanks!

Test it out with the following commands:

dart tool/hop_runner.dart build_debug
dart tool/hop_runner.dart build_release
dart tool/hop_runner.dart analyze_libs
dart tool/hop_runner.dart all_debug
dart tool/hop_runner.dart all_release