This change allows alternates to be stored in "$YADM_DIR/alt". The
correct path within the work tree will be symlinked.
Storing alternates within the work tree is still allowed. Both locations
will be considered when choosing an appropriate alternate file.
The new test system is written with py.test. These tests are more
comprehensive, run faster by an order of magnitude, and are far more
maintainable. The tests themselves conform to PEP8.
Tim Byrne