Fix #38: Correct trend diagram labels for decades <=1900
Fix #38: Correct trend diagram labels for decades <=1900

results.js: view.GraphResults.renderGraph:
- Render correct x-axis decade labels in the trend diagram for decades
  1900 and before by passing to Rickshaw.Graph.Axis.Time constructor the
  existing fixed time.ceil function with special handling of decades.
  (Previously, labels for decades 1900 and before were shifted back by
  one (1890 instead of 1900 and so on), probably resulting from
  Rickshaw's approximate handling of leap years.)
- Modify the fixed time.ceil to work for decades 1800 and before.

spraakbanken/korp-frontend,spraakbanken/korp-frontend,spraakbanken/korp-frontend