tests fail if xlwt is not installed.

[bmu]

./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'

[changhiskhan]

xlwt was accidentally introduced as hard dependency in PR #2370 (see comments from @y-p).
We'll fix.

[changhiskhan]

hmmm...that test seems to fail even with xlwt installed, and it's causing another test case to fail for me locally (PeriodIndex handling) because date parsing from Excel files seems to be failing.

[bmu]

all tests run on my system after installing xlwt

[changhiskhan]

Made xlwt a soft dependency