Added {{ content }} to post-index and home layout

Seems like something I might want to use (and it does not hurt to
have it there).
Added option to ex-/include categories to post-index

You can now specify lists page.included_categories and
page.excluded_categories (if included_categories is unspecified,
all posts will be included).
Changed default post preview to post.excerpt

Supporting the jekyll standard excerpt feature...
Added option for no gravatar...

...so that nothing is displayed when none is specified.