PortageGT 2.0.0

- Now uses SLOT 0 when available, only demanding a slot be specified if 0 does not exist
- Removed environment support (closes #9)
- New API, not backwards compatible (see issue #7)
- Fixed support for subslots (coses #8)
- Depends on Puppet 3.5.0 or newer
- Largely rewritten test suite