Fixed #4 - Cannot handle negative values

Had to defer the defaulting of the majorTicks array until after
the object has been set up, because we will not know what
the actual min/max values will be until then.
Re #4 - Cannot handle negative values

Adding support for floating values in the defaulting of the majorTick array.
Re #4 - Cannot handle negative values

Improved the formatting of default major ticks so that the format
of major ticks can be different form the format of the digital numbers.
Merge pull request #15 from rwblackburn/ticket4

Fixed #4 - Cannot handle negative values