nosetest errors when "xlrd" isn't installed
Maybe this is because I have openpyxl but not xlrd installed.

Anyway, I guess the tests should be skipped instead of throwing an error.

```
======================================================================
ERROR: test_parse_cols_int (pandas.io.tests.test_parsers.TestParsers)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/arthur/python-packages/pandas/pandas/io/tests/test_parsers.py", line 795, in test_parse_cols_int
    xls = ExcelFile(pth)
  File "/home/arthur/python-packages/pandas/pandas/io/parsers.py", line 1257, in __init__
    import xlrd
ImportError: No module named xlrd

======================================================================
ERROR: test_parse_cols_list (pandas.io.tests.test_parsers.TestParsers)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/arthur/python-packages/pandas/pandas/io/tests/test_parsers.py", line 813, in test_parse_cols_list
    xlsx = ExcelFile(pth)
  File "/home/arthur/python-packages/pandas/pandas/io/parsers.py", line 1257, in __init__
    import xlrd
ImportError: No module named xlrd

======================================================================
ERROR: test_to_excel_float_format (pandas.tests.test_frame.TestDataFrame)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/arthur/python-packages/pandas/pandas/tests/test_frame.py", line 3663, in test_to_excel_float_format
    reader = ExcelFile(filename)
  File "/home/arthur/python-packages/pandas/pandas/io/parsers.py", line 1257, in __init__
    import xlrd
ImportError: No module named xlrd

======================================================================
ERROR: test_to_excel_unicode_filename (pandas.tests.test_frame.TestDataFrame)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/arthur/python-packages/pandas/pandas/tests/test_frame.py", line 3693, in test_to_excel_unicode_filename
    reader = ExcelFile(filename)
  File "/home/arthur/python-packages/pandas/pandas/io/parsers.py", line 1257, in __init__
    import xlrd
ImportError: No module named xlrd

----------------------------------------------------------------------
Ran 2372 tests in 140.055s

FAILED (SKIP=18, errors=4)
```
