Better placeholder image while loading thumbnail
Allow title height to vary to handle long titles

A multi-line title will now have a properly shaded background on all
lines, not just the first one.
Add more air around title text
Use regexp for Chrome compatibility (fixes #2)

As explained on MDN[1], the use of a third argument for the
replacement flags is non-standard. Using a regular expression object
instead works across Firefox and Chrome.

[1]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/replace
Use correct sans serif font family name (fixes #3)

The font "sans" only worked on Linux since that system happens to have
a font with that name. I really wanted "sans-serif", which is the
generic font name for a sans serif font. It therefore also works in
Internet Explorer.
Add instructions for extract-thumbnail zapp