Add documentation for the ':wedge' style for arc.
Add missing ':center' style to arc.
Move adjustment for (left, top) if :center is set to a common routine.
Use new routine from arc, oval and rect
Added a 'center' option to Dimension init method.
Support :center in init_with_hash
If center is true, adjusts @left and @top by half of @width and @height respectively.
Updated Rect, Arc and Oval to pass whether :center is set in their opts hash.
Updated Dimension, Arc, Rect and Oval specs.
Updates as per feedback:
* Changed Dimension initialize method to take an options hash as the last parameter
* Renamed adjust_for_center as adjust_for_left_top_as_center
* Pass through of options hash from Shoes::Arc, Shoes::Rect and Shoes::Oval
* Added shared example to common_methods.rb "left, top as center"
Merge remote-tracking branch 'upstream/master' into arc-issue-423

Conflicts:
	lib/shoes/arc.rb
	lib/shoes/dimensions.rb
	lib/shoes/oval.rb
	lib/shoes/rect.rb
	spec/shoes/arc_spec.rb
	spec/shoes/dimensions_spec.rb
merge resolved
Dynamically compute left/top when centered not just at the beginning