Guard against empty list-items in base.tmpl

If the license/search_form are blank, don't show a list-item for them.
Fix spelling of 'prevlink'
Add Tim Chase to AUTHORS.txt
Merge branch 'clean_base' of https://github.com/Gumnos/nikola into lists

Conflicts:
	CHANGES.txt
	nikola/data/themes/base/templates/index_helper.tmpl
	nikola/data/themes/base/templates/post_helper.tmpl
Add issue #.
More guards.
Update literal.
Merge branch 'master' into lists-ramiro
Merge pull request #1040 from ramiro/lists-ramiro

#936 rebased & expanded