add a pre launch check for pub packages

@ussuri

- adds a pre launch participant for pub to check if all packages are installed.
- if not installed, shows a dialog with message to run pub get, and waits for user ok.
- continues with launch.

Added new method for package management. Not sure of bower usage, so don't know if we would want to pull out any of the other code into package management. If you think it useful, can do refactoring  in subsequent cl.

![screen shot 2014-06-05 at 2 40 18 pm](https://cloud.githubusercontent.com/assets/2192312/3194148/1538424e-ecfa-11e3-9333-a8e3b0309339.png)

changed to return bool - continue/cancel.