Implementing user-defined limits for ViewBox
Basic view limits appear to be working.
Removed duplicate limit-setting arguments
Renamed args for clarity, improved documentation
Fixed interaction bugs
 - zooming works correctly when view is against limit
 - no more phantom target range; target is reset during mouse interaction.
Wrap setLimits in PlotItem and PlotWidget
Merge branch 'viewbox_limits' into develop
Adds ViewBox.setLimits method

Conflicts:
	examples/SimplePlot.py