Extract category_link filter from category_generator.rb plugin

Allows category_link to be used as a filter to display a link to
any category.
Added robots.txt, with sitemap link
Titles in code plugins can have slashes in them. Closes #892
Removed unnecessary regex in code block plugin
Merge pull request #408 from dtchepak/category_link_filter

Extract category_link filter from category_generator.rb plugin
updated gems
Merge pull request #537 from joseprio/master

robots.txt generation