Make maximum value to measure always attainable

Fixes #68.
Improve whitespace in code
Allow for starting measurement without adjusted end value
Fix countdown algorithm

Previously, the amount of milliseconds since the last interpolation got
ignored, resulting in incorrect measurements (unless an update would be
done every millisecond?). Fixes issue #67.
Give percentages a bit more space

Fixes #63.
Work around node-canvas bug when rendering one full circle

Fixes #64.
Merge branch 'develop'