Accept Numeric values for dates.
override EXCERPT_ATTRIBUTES_FOR_LIQUID

Allow EXCERPT_ATTRIBUTES_FOR_LIQUID to be overridden by inheriting class. Right now will always reference Jekyll::Post::EXCERPT_ATTRIBUTES_FOR_LIQUID. This is already being used in the codebase for ATTRIBUTES_FOR_LIQUID.
Update Font Awesome to v4.1.0.
Header links to pages which have a title

Header used to add link to all pages of the site, even those which do not have any title, creating empty anchors on the page like `<a class="page-link" href="/feed.xml"></a>`. Those were non click-able and used space on the page due to margins.

Now only displays pages with a title.
Merge pull request #2410 from XhmikosR/master
Update history to reflect merge of #2410 [ci skip]
Merge pull request #2411 from mathbruyen/patch-1
Update history to reflect merge of #2411 [ci skip]
Merge pull request #2408 from quinn/patch-1
Update history to reflect merge of #2408 [ci skip]
Fix #2413 and setup a Jekyll.env.
date_to_xmlschema does some weird things with timezones, but whatever it's not important.
Merge pull request #2377 from jekyll/numeric
Update history to reflect merge of #2377 [ci skip]
Add jekyll.environment to site_payload.
Add test for Jekyll.env
Merge pull request #2417 from jekyll/jekyll-env
Update history to reflect merge of #2417 [ci skip]
Fix broken link on installation.md
Merge pull request #2421 from chezou/fix-broken-link