test_transform_broadcast failure on ubuntu 13.04
on ubuntu 13.04 test_transform_broadcast fails due to rounding issues.
I'm not sure if this is a bug in the test or if some underlying component may have changed to cause rounding errors when there were non before.
a possible reason might be numpy 1.7 because the test works fine in ubuntu 12.10 with numpy 1.6

see also https://launchpadlibrarian.net/122195559/buildlog_ubuntu-raring-i386.pandas_0.8.1-1_FAILEDTOBUILD.txt.gz
it affects git HEAD too.

```
test_transform_broadcast (__main__.TestGroupBy) ... > /usr/lib/python2.7/unittest/case.py(425)assertTrue()
-> raise self.failureException(msg)
(Pdb) bt
  /usr/lib/python2.7/unittest/case.py(332)run()
-> testMethod()
  /tmp/pandas/pandas/tests/test_groupby.py(404)test_transform_broadcast()
-> self.assert_((res[col] == agged[col]).all())
> /usr/lib/python2.7/unittest/case.py(425)assertTrue()
-> raise self.failureException(msg)
(Pdb) up
> /tmp/pandas/pandas/tests/test_groupby.py(404)test_transform_broadcast()
-> self.assert_((res[col] == agged[col]).all())

(Pdb) p res[col][0]
0.14593691347699592
(Pdb) p agged[col]   
0.14593691347699594
```
