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.
Correction: in the second case, the regex should be adjusted:

```dart
static final PACKAGE_REF_PREFIX_RE =
    new RegExp('^(../|.*/${bowerProperties.packagesDirName}/)');
url = url.replaceFirst(PACKAGE_REF_PREFIX_RE, '');
```