Specify an explicit pyusb version for pip 1.4 compatibility.

Fixed #18.
Remove error count from dashboard.

Once we got past very early testing, this became much less useful and
takes up valuable space.
Use reflection to pick up officially recognized measurements.
Add a bigger gap between dashboard columns.
Allow listening for *all* data wrapped in a Measurement.
Remove unused imports.
Support arbitrary non-numeric measurements wrapped in Measurement.
Calculate percent within the range of values seen for a measurement.
Reflectively initialize all known measurements to get units in output.
Update tests to support generic Measurement wrapper.
Add some color to dashboard to differentiate columns.