First pass at fixing issues identified in #423

Attempt at fixing issues raised in #423 

Added documentation to manual-en.txt for ```:wedge``` style for ```arc```.

Implemented missing ```:center``` stile for arc.

Couple of things to consider / I was unsure about:

* I patterned the implementation of the ```:center``` style after that implemented for ```oval``` (i.e. the code has been added to ```Shoes::DSL::arc```) - should really move it into a utility function (and then also use it within rect), but am unsure where to put it.
* I haven't worked out how to test the dsl within a spec yet to test ```:center``` - any pointers gratefully accepted :) (Could add a new simple example?)