Updated github mark for notmyidea

The smallest mark GitHub provides in the bundle is 32x32. Since they
explicitly say don't resize the mark, I asked them to provide a 16x16
so we could use it. I received the 16x16 in this commit from GitHub
support.

However, they suggested we use the 32x32 and use CSS rules to adjust
the size:

"...we use the 32px for both and employ css background and background-
size rules to handle retina displays. I recommend that technique,
since retina users will see a mess of pixels for icons that small."

This sounds like reasonable advice. Should we consider migrating the
notmyidea social icons to 32x32, with appropriate CSS to adjust the
sizes?
Updated tests
32px Github logo and CSS shinker
Use absolute size for github mark
HTML error in notmyidea: <li> not allowed in <section>.

<li> tags need to be inside of <ul> or <ol>.
Thanks to the w3c_validate plugin for finding this.
Fix stray </ol> and </section> if only one article is displayed.

We already check if loop.length > 1 before outputting <section> and <ol>
tags, but we neglected to do the same check when outputting the corresponding
end tags.

Also, since I had to read the code when I touched it, simplified a conditional:
if (a) if (a and (b or not b and c))
can be simplified to
if (a) if (b or c)

Note the "b or not b", it was just too ugly to not fix.
Some browsers like Lynx render adjacent links without implicit spaces.

Put an extra space at the end of each link to a tag so that Lynx doesnt render
the tags as a single word.
Merge pull request #1265 from mlang/valid-html

Fix notmyidea theme's HTML tags
Merge pull request #859 from cdunklau/new_github_logo

Update GitHub icon for notmyidea theme
Run tag name through escape filter to avoid invalid HTML

If a tag contains characters like <> or &, we currently generate invalid HTML.
This is easily fixed by sending the tag through the jinja escape filter.

(This bug is not theoretical, I hit it when using C++ template names for tags,
 like "boost::variant<>".)
add lang support for drafts (#826 & #1107)

Fix #826
Fix #1107
Merge pull request #1201 from hrbonz/draft_lang

Add language support for drafts
Merge pull request #1267 from mlang/escape-for-html

Run tag names through escape filter to avoid invalid HTML
Minor improvements to Settings docs
Add the setting SLUGIFY_ATTRIBUTE
Merge pull request #1248 from th3aftermath/AddSlugifyAttribute

Add the setting SLUGIFY_SOURCE. Refs: #197
document concept of caching and generating only modified
first working concept of content caching
concept of caching output context

- moved caching methods to the CacheManager class
- comparing context dicts does not work well,
  because Article, etc. classes don't have
  __eq__ implemented
add --rebuild-cache cmdline option and extend docs