Optionally report failures in a JUnit-compatible XML.

Implements part of #2, allowing better monitoring of devpi-builder invocations on Jenkins.
PEP8
Test refactoring

Extracted assertion logic for test cases in junit.xml files.
Optionally report skipped packages in a JUnit-compatible XML.

Implements part of #2, allowing better monitoring of devpi-builder invocations on Jenkins.
Optionally successfully built packages in a JUnit-compatible XML.

Completes implementation of #2, allowing better monitoring of devpi-builder invocations on Jenkins.
Fix build issues on Python 3.2

junit-xml from Cheeseshop is broken on Python 3. Use a custom, known-to-work version from Github.
Fix handling of build errors on Python 3.
Update list of features in documentation.
Updated help on command line arguments.
Merge pull request #5 from blue-yonder/junit_output