MRG: Cleaner tests

In the process of working on #253 I was a little bit surprised by the number of warnings during testing. This cleans that up, and fixes a couple minor errors along the way.

Ready for review/merge.
e.g.:
```
larsoner@bunk:~/custombuilds/nibabel$ make testmanual
F..F
======================================================================
FAIL: Doctest: coordinate_systems.rst
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/usr/lib/python2.7/doctest.py", line 2226, in runTest
    raise self.failureException(self.format_failure(new.getvalue()))
AssertionError: Failed doctest test for coordinate_systems.rst
  File "/home/larsoner/custombuilds/nibabel/doc/source/coordinate_systems.rst", line 0

----------------------------------------------------------------------
File "/home/larsoner/custombuilds/nibabel/doc/source/coordinate_systems.rst", line 58, in coordinate_systems.rst
Failed example:
    plt.suptitle("Center slices for EPI image")
Expected nothing
Got:
    <matplotlib.text.Text object at 0x2b0f27e16790>
```