TST: make floats / ints explicit for rst tests

Python 3.2 unhappy if we format for float but it's a numpy int.  Make numpy
dtypes explicit in tests to work round this.

Closes gh-246, I hope.
doesn't work on that elderly ubuntu either:
```
root@head2:~/nibabel-1.3.0+git466-g27e4feb# nosetests3 --pdb -s -v /tmp/buildd/nibabel-1.3.0+git466-g27e4feb/nibabel/tests/test_rstutils.py
nibabel.tests.test_rstutils.test_rst_table ... FAIL

======================================================================
FAIL: nibabel.tests.test_rstutils.test_rst_table
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/nose/case.py", line 198, in runTest
    self.test(*self.arg)
  File "/tmp/buildd/nibabel-1.3.0+git466-g27e4feb/nibabel/tests/test_rstutils.py", line 70, in test_rst_table
    +--------+--------+--------+--------+--------+"""
AssertionError: '+--------+--------+--------+--------+--------+\n|        | col[0] | col[1] | co [truncated]... != '+--------+--------+--------+--------+--------+\n|        | col[0] | col[1] | co [truncated]...
Diff is 756 characters long. Set self.maxDiff to None to see it.

----------------------------------------------------------------------
Ran 1 test in 0.005s

...
(Pdb) print first
'+--------+--------+--------+--------+--------+\n|        | col[0] | col[1] | col[2] | col[3] |\n+========+========+========+========+========+\n| row[0] | 0.0    | 1.0    | 2.0    | 3.0    |\n| row[1] | 4.0    | 5.0    | 6.0    | 7.0    |\n| row[2] | 8.0    | 9.0    | 10.0   | 11.0   |\n+--------+--------+--------+--------+--------+'
(Pdb) print second
'+--------+--------+--------+--------+--------+\n|        | col[0] | col[1] | col[2] | col[3] |\n+========+========+========+========+========+\n| row[0] | 0      | 1      | 2      | 3      |\n| row[1] | 4      | 5      | 6      | 7      |\n| row[2] | 8      | 9      | 10     | 11     |\n+--------+--------+--------+--------+--------+'
```