gravatar: fix generated HTML to validate correctly

This introduces a simple alt="" value which is required by the new
HTML5 validation, and also fixes the URL to use the correct &amp;
syntax.
feeds: don't append the subtitle to the page title.

Since the subtitle is already passed to the feed itself, repeating it
here can only cause redundancies. Furthermore with the current version
of Typo, it's possible to change the format of the pages' titles.
akismet: replace vendored, hacked, old akismet version with 1.0.0

The new version can be loaded through Gemfile instead and might work a
little better.
Include tip on how to add single and multi-word tags to posts

Clearly detail how to properly separate multiple tags on a post
by the use of commas. Follows convention mentioned in help section
of SEO::Titles.

If a user needs to add multi-word tag, then it should also tell
them how to do so by use of double quotes. Otherwise they may
not expect to see it broken up into individual words as spaces
also can separate tags.
Merge pull request #144 from randomecho/article-tagging

Include tip on how to add single and multi-word tags to posts
Merge pull request #143 from Flameeyes/master

Small fixes
Extract secure token generation to a service object
Clean up admin/tags_controller spec
Use generic template logic for grouping template overriding
Simplify SEO option specs