Use Dart's default lazy instantiation for the scheduler

Dart will lazily instantiate final and constant fields. More info [here](http://news.dartlang.org/2012/02/static-variables-no-longer-have-to-be.html).
Action : consolidate to a single constructor

fixes #10
Ignore the packages folder in sub-directories

Dart creates a packages folder in the example/ and test/ directories. These probably shouldn't be committed to source control.
Merge pull request #13 from rmsmith/unify_action_ctor

Unify action ctor