[CDF-405] -  CCC - Options to control the minimum and maximum precision/units of temporal ticks

* Protovis update.
* Cartesian continuous axis
  * New options TickUnit, TickUnitMin and TickUnitMax.
  * Temporal axis now also support LabelSpacingMin.
  * Handling edge cases of 1,2,3 tick counts.
  * Option DesiredTickCount now supports value Infinity, meaning: use as many ticks as can fit without overlapping (this is the default for numeric ticks).
  * Options DesiredTickCount and LabelSpacingMin are now both used; Previously, when DesiredTickCount was specified, LabelSpacingMin was ignored.
  * Slight relaxing of the label trimming decision
  * Slightly increased the effective textHeight value used for measure calculations; was affecting adversely the minimum label spacings decisions. Unfortunately, this causes old labelSpacingMin values to result differently.
  * Fine tuning labelSpacingMin's default value, for better backward compatibility
  * Fixed some "minimum label spacings" calculations.
* Fixed issue in SVG generation, with NaNs; occurred in the grid rules, when the main plot panel would not be visible, due to there being no space available.

@pamval please review.