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
Initial commit of relative dimensions support

This will resolve issue #380. The Dimensions class needs an update to draw the
width and/or height from the parent, which now needs to be passed in to work
properly.

For the time being I've chosen to make it graceful if an object doesn't pass
the parent, but this might not be necessary once all the DSL elements properly
support the parent.
resolved merge from master
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.
Merge pull request #1 from PragTob/jasonrclark-relative_dimensions

Merge in master branch
Shared examples for dimensioning
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"
Apply shared dimensions spec throughout DSL
Potential fix for Shoes::CheckButton dimensions

Turns out that both Shoes::Button and Shoes::CheckButton derive from SwtButton, but
only Shoes::Button's DSL was being allowed to set widths appropriately here.
use slot#add_child instead of direct variable access everywhere
Basic implementation of Slot#prepend
Further dimension additions
Don't type-check in SwtButton
Merge branch 'master' into prepend
Extensive Slot#prepend specs
Introduced SlotContents to take care of prepend logic
Merge pull request #451 from PragTob/prepend

Basic Slot#prepend implementation
Fix for relative dimensions on star