Use system font, not the normal Helvetica Neue Light

Yosemite has /System/Library/Fonts/HelveticaNeue.dfont and
/System/Library/Fonts/HelveticaNeueDeskInterface.ttc. The font names
in the latter are different (also note the leading dot), and revealed
using mdls:

[...]
com_apple_ats_name_fond        = (
    ".HelveticaNeueDeskInterface-Bold",
    ".HelveticaNeueDeskInterface-BoldItalic",
    ".HelveticaNeueDeskInterface-Heavy",
    ".HelveticaNeueDeskInterface-Italic",
    ".HelveticaNeueDeskInterface-Light",
[...]

The fonts look slightly different and have different metrics.
More modernization

The duplication in the cover image code is removed.
Merge pull request #39 from chrisridd/master

Tweak the font, more modernization