Added build.dart to make Polymer iterations in Dart Editor easier.

Essentially just pulls in the changes under widgets/ into ide/app/packages, allowing quick debugging of the uncompiled app.

Still needs to be run manually every time 'widgets/' change, but that can be done via the context menu or Run button of the editor without switching to command line and back.
Addressed comments
Merge pull request #110 from ussuri/add-build-dart

Added build.dart to make Polymer iterations in Dart Editor easier.