ext/latex_mods: Fix TypeError for alt tags

When not using alt tags on figures, latex_mods.py failed with a TypeError. The
problem was, that the strings are not necessarily unicode, but the escape_map
used only works with unicodes.

Fixes #12

Signed-off-by: Erik Bernoth <erik.bernoth@gmail.com>
more verbose requirements in docs

Signed-off-by: Erik Bernoth <erik.bernoth@gmail.com>
Merge pull request #15 from erikb85/fix-shortcaptionerror

ext/latex_mods: Fix TypeError for alt tags
Merge pull request #10 from erikb85/master

small cleanups
Merge pull request #14 from erikb85/readme

more verbose requirements in intro
Merge remote-tracking branch 'jeff/master' into master