Enables setting a percentage-based center coordinate while resizing

As an alternative to anchor points, this allows you to set a
center=0.5,0.8 point, which in this case would be slightly off the
center of the image. It allows you to translate image focal points to
automated crops.
Tweaking the center calculation abit
Fixes #31
v1.8.4
Merge remote-tracking branch 'origin/dev'

Conflicts:
	src/ImageProcessor.Web/NET45/Properties/AssemblyInfo.cs