Merge pull request #447 from aaugustin/load-panels-contents-on-demand

 Load the content of panels dynamically
Avoid monkey-patching when it isn't necessary.
Pass through attributes lookups in CacheStatTracker.

Fix #439.
Fix confusion between __getattr__ and __getattribute__.
Remove Panel.disabled, always use Panel.enabled.
Disable instrumentation for disabled panels.

This eliminates the performance overhead when a panel is disabled.
Remove code that isn't used any more.
Add test for the cache panel.