restructure example files

Enable a larger number of examples by moving each of them into subfolders.

Adapt gulp tasks to include all example files.

`src/index.html´ is meant as an introductory website with pointers to
documentation and specific examples.

Changes on that page trigger connect server reload while running `grunt
serve`.
two new examples

These examples are not finished, yet.
They do reveal some new issues, which will be
tackled in later commits.

* colors - player surrounded by different background colors (unfinished)
* cre-206 - real world podcast with metadata
* picture-stream - media with an picture track/stream (unfinished)
* video - example with a video as the main media (with issues)
WIP - picture-stream example
WIP - video example
mark video example as in progress
Merge pull request #166 from podlove/more_examples

restructure example files