Use Adobe Blank as fallback for all previewed fonts.

I was looking for unicode coverage of some chars (`▶▼ | ▸▾ | ►`) which are missing from most fonts - but I was seeing them under every font, due to system fallback to other fonts.  This snould fix that.

Demo: https://rawgithub.com/cben/iosfonts/blank-fallback/index.html
Screenshots with iOS 4.3:
https://saucelabs.com/jobs/97e0ec1c7e054c4bbf77b0ec3d075a29/0001screenshot.png
https://saucelabs.com/jobs/97e0ec1c7e054c4bbf77b0ec3d075a29/0002screenshot.png

A side effect: the website now look nearly empty on non-Apple platforms
(on ubuntu I see Courier & Helvetica, everything else is blank).
But that's arguably a feature as viewing the site on when you don't have the fonts is misleading.

Ideally should use a non-empty placeholder for missing characters.
But placeholder fonts such as [Unicode BMP Fallback or Apple's LastResort](https://en.wikipedia.org/wiki/Fallback_font) weigh 4-5MB compared to Adobe Blank's 30KB...
It's probably possible to mention them in the font stack when existing on the system — notably OSX — but I don't think iOS has LastResort and different behavior between OSX and iOS may be more confusing than useful...
(It should also be possible to use Adobe Blank's technique to create a compact font using one non-empty glyph for all unicode.  But I'm don't care *that* much.)

----

While I'm at it, mention font licenses in README.

 - I'm [not fully sure](https://github.com/danielbruce/entypo/issues/22) if Entypo's license is CC BY-SA or SIL OFL.  But Font Squirrel, Fontello & IcoMoon all say BY-SA.
  Also, my mention in README that it was processed by Font Squirrel is a guess, please fix if wrong.

 - There is fontello.css and both it and style.css mention css/font/fontello.ttf and friends but those doesn't exist and AFAICT the "fontello" font family is never used?  So I didn't mention it.