Devoncarew stagehand

Use stagehand for some of our dart project templates:

- change the dart web template over to the one in stagehand
- add stagehand's dart package template
- remove the dart polymer app template - it generally doesn't run for users w/o them knowing that it has be run in Dartium

also
- created a new `StagehandProjectTemplate` class
- moved the existing templates to be subclassed by a new `SparkProjectTemplate` class
- made `ProjectTemplate` be an abstract, mostly empty class

@ussuri 
Not sure if it's that important and/or achievable with the current architecture, but my gut instinct originally was to keep specific template implementations in `template.dart` sources directly under matching subdirectories, which would mean `templates/stagehand/template.dart` in this case (and a new subdir for SparkProjectTemplate above). That's just an opinion, though, and I don't want it to block this CL. So never mind :)