add hyperlink support for bower references

@ussuri, add hyperlink support (command-click) to bower references in the source code. When working to convert the polymer sample over to paper elements, this was one thing I found lacking.
How about:

```dart
final PACKAGE_REF_PREFIX_RE =
    new RegExp('^(../|.*/${bowerProperties.packagesDirName}/)(.*)$');
...
Match m = PACKAGE_REF_PREFIX_RE.matchAsPrefix(url);
if (m != null) url = m.group(2);
```