Complete Scales API for missing methods

[anthonime]

• interpolate methods to be implemented as soon as we cover the interpolators and D3.interpolate methods. POSTPONED TO LATER VERSION
• log.tickFormat for String and function seems to be bugged... see log.tickFormat code snippet is wrong d3/d3#1384
  We need to implements the scales.interpolate() methods
• recheck the methods ticks in linear, log and pow since it seems to have 2 version : without and with a count arg, and documentation is inconsistent between all those scales
• recheck the methods tickFormat in linear, log and pow since it seems to have 2 version : without and with a count arg, and documentation is inconsistent between all those scales
• see also Upgrade to 3.2.x version #63 (invertExtent)

[anthonime]

first bullet has been postponed to #75