This makes it possible to run e.g. "yadm -Y foo introspect repo
2>/dev/null" and get an empty output instead of getting the error
message about foo not being fully qualified.
* Create a YADM_DATA base (that is the source for repo & archive)
* Add --yadm-data to override
* Default YADM_DATA will be determined by XDG_DATA_HOME
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.
