cleanup

remove semicolons only present on those lines
update warnings suggested by Xcode 5

* add warnings now enabled by default on new projects
* add new warning flags

Some of the new warnings are enabled by YES_ERROR in Xcode 5,
but unfortunately this resolves to NO in Xcode 4.x. But as we have enabled
-Werror by default, this should not matter.
add warning for repeated use of weak refs
Merge pull request #7 from diederich/Xcode5Warnings

Update Common.xcconfig with warnings suggested by Xcode5