Sprite: Allow image name with dot

@am11 : Can you check the fix (as you develop it on 01ce1eba4c4870f98fa99f1c3f71a0fc599828a0), I choose to not use the [FileHelpers.GetFileNameWithoutExtension method](https://github.com/madskristensen/WebEssentials2013/blob/2e93ca5079e2cf9f982ae25b8c13ca5bd5c36411/EditorExtensions/Shared/Helpers/FileHelpers.cs#L438) because I'm not sure to understand to exact purpose of this method compare to the Path.GetFileNameWithoutExtension method of the framework. I tested it and everything works well but just to be sure that we don't break something for a bug that can be bypass with an easy workaround.

This will close #1517 , after Adeel has checked.