Making Column implement HasAlignment so users can specify the default alignment of cells in a column.  In the future, we may add a way to specify the alignment of specific Cells.
Issue: 5623

Review at http://gwt-code-reviews.appspot.com/1134801

Review by: rchandia@google.com

git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@9269 8db76d5a-ed1c-0410-87a9-c151d255dfc7