Running group should run group:default if it exists and no group task exists

[kitgrose]

Ideally phake would use the following order of precedence to determine which task to run:

1. A task with the exact name within the current or named group
2. The default task in the current group
3. Error

That means if I have tasks for default, build, run, test, deploy, db:init, db:update, db:delete and db:default, test:init, test:run, and test:default, the following behaviour is expected:

1. Running phake run would execute run
2. Running phake default would execute default
3. Running phake would execute default
4. Running phake db:init would execute db:init
5. Running phake db:default would execute db:default
6. Running phake db would execute db:default
7. Running phake test:init would execute test:init
8. Running phake test:default would execute test:default
9. Running phake test would execute test (there's a named task called test which has higher precedence than test:default)

As far as I can tell, that mimics the behaviour of the root :default task fairly closely without breaking compatibility with existing Phakefiles.