change cbook to relative import

In the current master, the following bug is present in python 3
```
 $ python3
Python 3.2.3 (default, Oct 19 2012, 20:10:41) 
[GCC 4.6.3] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import pylab
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/local/lib/python3.2/dist-packages/pylab.py", line 1, in <module>
    from matplotlib.pylab import *
  File "/usr/local/lib/python3.2/dist-packages/matplotlib/pylab.py", line 219, in <module>
    from cbook import flatten, is_string_like, exception_to_str, \
ImportError: No module named cbook
```

This changes `cbook` to a relative import, which works in both python 2.6+ and python 3+.