Complete archive tests. Not yet exactly sure how to test for filesystem failures.
Write tests for checkout.
Raise coverage to reasonably attainable levels. 

Mostly filesystem-related errors and other stuff that's tedious to simulate 
are left uncovered.
Switch on branch coverage.  Refine testing.

Exclude raised exceptions from coverage.
Uncovered branches left are either due to a bug in coverage 
(https://bitbucket.org/ned/coveragepy/issue/146/) or belong to not-yet 
covered code.
Reached 96% coverage, 8 missing statements, 8 partial branches.
Update Readme. Let's call this version a beta 0.5.