tests fail if xlwt is not installed.
```
./test_rebuild.sh 
running clean
running build_ext
cythoning pandas/src/generated.pyx to pandas/src/generated.c
building 'pandas._algos' extension
....
E
======================================================================
ERROR: Failure: ImportError (No module named xlwt)
----------------------------------------------------------------------
Traceback (most recent call last):
File "/net/home4/bmueller/.virtualenvs/myenv/lib/python2.6/site-packages/nose/loader.py", line 390, in loadTestsFromName
addr.filename, addr.module)
File "/net/home4/bmueller/.virtualenvs/myenv/lib/python2.6/site-packages/nose/importer.py", line 39, in importFromPath
return self.importFromDir(dir_path, fqname)
File "/net/home4/bmueller/.virtualenvs/myenv/lib/python2.6/site-packages/nose/importer.py", line 86, in importFromDir
mod = load_module(part_fqname, fh, filename, desc)
File "/scratch/pandas/pandas/__init__.py", line 33, in <module>
from pandas.io.parsers import (read_csv, read_table, read_clipboard,
File "/scratch/pandas/pandas/io/parsers.py", line 9, in <module>
import xlwt
ImportError: No module named xlwt
...
Ran 1 test in 0.002s

FAILED (errors=1)

In [3]: pandas.version.version
Out[3]: '0.10.0.dev-504c7f1'
```
