Adjust copy-examples build task to package.json

This custom build task copies snippets and examples to the build folder.

- adjust to reading package.json instead of index.json
- rename the task to "copy-examples"
- use path.join() instead of string concatenation
- copy README.md files in snippets folder as well if it exists
- add console output

Change-Id: I3d1de52d577b5de6d2ed87184363097fa1ed845a