Add ability to prune old screens
Simplify readability of Minitest condition
Ensure pruning occurs for all test suites when pruning enabled
Move pruner_spec into unit tests for consistency

All test that are not specific to a framework and invoked with Aruba are currently contained within /unit
Simplify the Pruner class & tests
Test that prune is only called once per test suite for performance reasons
Pruning documentation
Ruby 1.9 support
Ruby style improvements for consistency
Make RSpec prune tests more reliable by setting the modify time
Version bump to support pruning
Merge branch 'feature/prune-last-screens'