Image can now be painted and navigated

Also fixes #31 where axis numbers weren't scaling properly.
Axes are now invertable

Axes can be inverted with the properties:
xAxis.inverted = true
yAxis.inverted = true
Image support implemented

Navigation with mouse still needs testing.
New properties for Axis: fitPlots, aspectRatio
Fixing mouse nav for fixed aspect ratio
Fixing merge conflicts
Merge branch 'imageSupport'

Bring imageSupport to master
Implemented Grid Lines for axes

Grid lines can be enabled for X and/or Y axes.
The style can be set using the LineSpec grouped property.