Memcached panel

[stevepiercy]

Add a new third party panel for Memcached. See #110.

Additionally promote via:

• Add to https://docs.pylonsproject.org/projects/pyramid-debugtoolbar/en/latest/#panel-and-ui-extras
• Add listing to TryPyramid.com - Extending Pyramid. See Template to add items to Extending Pyramid trypyramid.com#202 for details.
• Tweeting from @PylonsProject. Ping @stevepiercy.
• Author may post to https://groups.google.com/forum/#!forum/pylons-discuss