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
• [x] log.tickFormat for String and function seems to be bugged... see https://github.com/mbostock/d3/issues/1384 We need to implements the scales.interpolate() methods
• [x] 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
• [x] 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
• [x] see also #63 (invertExtent)

[anthonime]

first bullet has been postponed to https://github.com/gwtd3/gwt-d3/issues/75