change Polymer startup code like it should be (to my knowledge)

Hi MarkHats!

I responded to your issue on dartbug.com (https://code.google.com/p/dart/issues/detail?id=17467).

While playing with your code I made some changes:
(I thought a PR is the easiest way to communicate this to you - no need to feel any pressure to apply any of the changes I made)

- I deleted the build.dart files (no idea if this changes anything but I don't use them anymore since a while myself and I just wanted to ensure that nothing is interfering)

- I removed all version constraints in both pubspec.yaml files to ensure that I get the most recent versions.

- I removed the entrypoint in the widget.dart package.  
Polymer transformer configurations in imported packages cause trouble (to my knowledge) this should be fixed soon with dev_transformers, but a few days ago it still caused me troubles in another project.

- I had to change the path separator `\` -> `/` to make it work on my Linux machine

-  web/polymerbootstrap.dart
  - this is how a custom `main()` should look like with Polymer (to my current knowledge, but might be wrong/not necessary, ...)

-  web/polymerbootstrap.html
  - line 9 in polymerbootstrap.html (`<link rel="import" href="packages/polymer/polymer.html">`) is for Polymer 0.10.0-xxx
  - this is how the scripts in the entry page should be organized for Polymer apps (to my current knowledge, but might be wrong/not necessary, ...)
  - I removed the jQuery/Bootstrap JavaScript links because they caused exceptions when running as JavaScript.

Hope it helps.
I again learned a few things while investigating the problem.
Cheers!
Günter