added sort by year example

#1

features/sorting.feature

+Feature: Sorting BibTeX Bibliographies
+  As a scholar who likes to blog
+  I want to sort my bibliographies according to configurable parameters
+
+	Scenario: Sort Bibliography by Year
+		Given I have a configuration file with "citation_sort_order" set to "year"
+	  And I have a page "references.bib":
+			"""
+			---
+			---
+			@book{ruby,
+			  title     = {The Ruby Programming Language},
+			  author    = {Flanagan, David and Matsumoto, Yukihiro},
+			  year      = {2008},
+			  publisher = {O'Reilly Media}
+			}
+			@book{ruby,
+			  title     = {The Ruby Programming Language},
+			  author    = {Flanagan, David and Matsumoto, Yukihiro},
+			  year      = {2007},
+			  publisher = {O'Reilly Media}
+			}
+			"""
+	  When I run jekyll
+	  And I should see "\(2007\)\.+\(2008\)" in "_site/references.html"