First stab at twitter cards.

BIG WARNING: please see if there can be escaping problems with the
<meta> tags (mostly the abstract).

Note that MetaCPAN will have to apply at
https://dev.twitter.com/form/participate-twitter-cards

There is still the module card to do, and I'm sure the elements
of the cards for the author and release could be tweaked to be
even better.

If we want to add author information to the release card, we might want
to fold the twitter_card logic in the controller, which might
not be a bad idea at all because if we do that, the TT code will
simply become


  <% IF twitter_card %>
  <% FOREACH item IN twitter_CARD %>
  <meta name="twitter:<% item.name %>" value="<% item.value %>" />
  <% END %>
  <% END %>
twitter cards are dealt with in the controllers
Revert "twitter cards are dealt with in the controllers"

This reverts commit 9c1d171a82debf631c1765e1617a40daef3cd288.
add module twitter card
Merge pull request #783 from yanick/twitter-cards

First stab at twitter cards.