sourcedr: Implement project config and commands

This commit implements project configuration files and rework the
command line interfaces.  Now, the command line usages are:

```
sourcedr init [android-root]
sourcedr scan
sourcedr review
```

Test: ./sourcedr/functional_tests.py

Change-Id: I41b28bb30f8adda3754d72204e4ce3e7da84b0b1