Fixed subdirectory issue.

I changed the structure to put code in ./code, but that didn't work for preface,
whish is not in a subdirectory (parallel to ./code).

code/book_format.py

@@ -2,8 +2,8 @@
 from IPython.core.display import HTML
 import matplotlib.pylab as pylab
 
-def load_style():
-    styles = open("../styles/custom2.css", "r").read()
+def load_style(name='../styles/custom2.css'):
+    styles = open(name, 'r').read()
     return HTML(styles)
 
 pylab.rcParams['lines.linewidth'] = 2