Fix white gap between image and border

Fixes #218. I have manually tested this with the 'qt4.image', 'qt4.qpainter', and 'wx.image' backends on OSX.

I seen elsewhere the center-pixel adjustment used here, but I am uncertain about the details of why it is needed. For example, the `ColorBar` does not need this adjustment. (Is there something wrong with the `kiva.agg.GraphicsContextArray` used in the `ImagePlot`?)

@rkern, would you mind reviewing this?
You will need to try other GCs to be sure that this is the right place to do this adjustment. The different backends can behave differently, albeit unintentionally. We have been fighting these off-by-(one/half) issues for a long time.