Caching added to #articles and #sorted_articles

Added caching for #articles and #sorted_articles to speed up for may items.
only cache when preprocessing is ready

Only cache these values if preprocessing is ready. Preprocessing is ready when the
@items attribute is frozen.
removed unneccessary line for returning instance variable
Silence "Could not find files for the given pattern(s)" message on Windows
Add release notes for 3.6.3
Update version to 3.6.3
Improve notification tool detection
Merge branch 'bug-quiet-watch-on-windows' into release-3.6.x

Conflicts:
	lib/nanoc/cli/commands/watch.rb
Update release notes
Teardown listeners after compile
Setup listeners only once
Allow injecting listener classes
Add setup/teardown listener test case
Clear up #enable_for? documentation
removed doubled block for #articles
removed doubled block for #sorted_articles
Merge pull request #269 from matthiasbeyer/blogging-helper-caching

Cache #articles and #sorted_articles
Merge pull request #301 from ddfreyne/bug-teardown-listeners-after-compile

Teardown listeners after compile
Update release notes
Merge branch 'release-3.6.x'

Conflicts:
	lib/nanoc.rb