BUG: Fix to read decimal seconds from Excel.
Fix to allow decimal seconds to be read from Excel dates and times
into datetime objects. #5945.

This required a fix to the `xlrd` module to return milliseconds from Excel dates and times. That fix was recently released to PyPI in xlrd version 0.9.3.

Tests, version updates and release note included.

this needs to be able to raise on an older version of `xlrd` (and a test that will correctly skip) on the same.
 @jreback 

Patch updated with your comments, squashed and re-pushed:
- I made the `ci/requirements-3.2.txt` change to `xlrd` version 0.9.2 for testing (although I'd prefer to change that back to 0.9.3 before merging since it is bound to cause confusion because of inconsistency).
- I made the `LooseVersion()` changes in `pandas/io/excel.py` and `pandas/io/tests/test_excel.py`.
- I tested with an older version of `xlrd`. The tests all skip and the module throws an exception:

``` python
>>> import pandas as pd
>>>
>>> pd.read_excel('decimal_seconds_1900.xls', 'Sheet1')

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "pandas/io/excel.py", line 108, in read_excel
    return ExcelFile(io, engine=engine).parse(sheetname=sheetname, **kwds)
  File "pandas/io/excel.py", line 131, in __init__
    "support, current version " + xlrd.__VERSION__)
ImportError: pandas requires xlrd >= 0.9.3 for excel support, current version 0.9.2
```
 @jmcnamara see my comment above. The ci tests multiple versions of every library so that back compat can be maintained. 0.9.2 should work unless I need to the new capability. Its not realistic to force a user to upgrade `xlrd` unless they actually need to use the new capabilities (because even though its the current pypi release, what's going to happen is people may not upgrade to it at the same time as they upgrade pandas or vice-versa).
 @jrebeck

Ok. Fair enough.

I'll revert most of the changes and just put a conditional in the date conversion to use the millisecond method if available or use the 0.9.2 method if not.
 I've reworked the PR to handle millisecond times when `xlrd >= 0.9.3` is available or to use the previous method if not.

I've added a test case for both conditions and also for the twin Excel epochs for good measure.
 hmm...it is skipping on 3.2 for a different reason (xlwt is not installed). so not running your test. https://travis-ci.org/jmcnamara/pandas/jobs/23636210

so really only testing excel in 2.7...(the locale build is not even running the tests)...

let me add these deps to 2.6

hold a sec
 ok...rebase on master...I added the deps to the 2.6 build; so should skip correctly there (the 3.2 build doesn't really matter) because almost all tests that need to do writing are skipping anyhow (because xlwt is not installed)
 Rebase done so let's see what Travis says.

I tested locally with xlrd 0.9.2 and 0.9.3 and both passed.
 looks good...thanks
