Add new option for controlling whether to display categories on menu or not

Conflicts:
	docs/changelog.rst
	pelican/tests/output/basic/tag/bar.html
	pelican/tests/output/basic/tag/baz.html
	pelican/tests/output/basic/tag/foo.html
	pelican/tests/output/custom/tag/bar.html
	pelican/tests/output/custom/tag/baz.html
	pelican/tests/output/custom/tag/foo.html
Merge pull request #578 from davidjb/display-categories-option

Add option to control displaying categories on menu
unify css class for pygments
Merge pull request #824 from avaris/markdown-css

unify css class for pygments
fix for issue #761: handle unicode correctly in summary for Markdown
Merge pull request #825 from avaris/markdown-summary

fix for issue #761: handle unicode correctly in summary for Markdown
Fix for issue #428: Use PLUGIN_PATH properly
Merge pull request #828 from avaris/plugin-path

Fix for issue #428: Use PLUGIN_PATH properly
Minor English grammar correction