Added support for specifying SharedSizeGroup

[markhewett]

Fix for issue #7. I needed SharedSizeGroup support, so I hacked together a way to specify the group name in the Columns and/or Rows properties. You can specify a shared size group by adding the name in parentheses, e.g.

<ag:AutoGrid Columns="Auto(GroupA),100,100" Rows="Auto">...

Seems to work OK for my needs, and doesn't seem to break the example application, but I have not run through any extensive testing. Just wanted to throw this out there in case it is useful for others.

[carbonrobot]

Interesting approach, I will take a look at this.