Refactored Executor code
Improved sample code
Replaced "0store-win purge" with "0install-win store purge"
Made FrontendCommand inherit from Resolver instead of containing an instance of it
Setup: Replaced ZeroInstall.exe with "0install-win central" and 0store-win.exe with "0install-win store manage"
FrontendCommand unit tests: Improved Handler mocking