tests: excise unittest2 dependency

Given that pytest is already pretty much here to stay without
significant work, write these tests to use pytest's functionality to
be able to avoid unittest/unittest2.

Unfortunately this seems to require a change of behavior in how the
cause of the skip will be reported.

Signed-off-by: Daniel Farina <daniel@heroku.com>

Conflicts:
	tests/test_s3_worker.py
Fix upper case bucket names regression (from #3).

Evidently, boto.storage_uri called without extra connection arguments
constructed a connection object compatible with uppercase bucket names
(which I discovered today is only allowed in us-east-1; one gets an
error attempting to create a bucket with an uppercase name in all other
regions).  Issue #3 added a 'host' connection argument that appears to
have changed the configuration.  Explicitly request the appropriate
calling format.

Signed-off-by: Daniel Farina <daniel@heroku.com>