There are at least two extensions that can do this. One is CodeMaid which explicitly will trim trailing blanks on save, and the other is Productivity Power Tools which can run the Format Document automatically on save.
To add an extension from within Visual Studio 2012, select the
Tools menu, then Extensions and Updates.., then in the window select Online on the left and enter your search string for CodeMaid or Productivity Power Tools in the box on the upper-right.