TST: Tests and fix for unhandled data types in Excel writers.
Added tests and a fix for unhandled numpy data types in the
Excel writers. Issue #3122.

Some background to this PR:
1. The original issue #3122 is due to the fact that xlwt whitelists the types that it handles and raises an exception for unhandled types. The fact that certain `numpy` types aren't identified correctly may be a `numpy` bug but their devs have rejected that as an issue.
2. The `pandas/io/excel.py` module contains a `_conv_value()` method to map pandas/numpy types to types handled by xlwt/openpyxl.
3. This PR extends/strengthens the `_conv_value()` function to handle more generic types.
4. This PR adds more extensive, although non-exhaustive, tests for `numpy` [data types](http://docs.scipy.org/doc/numpy/reference/arrays.scalars.html).
5. XlwsWriter doesn't require the `_conv_value()` method since it has its own type conversion methods. So I've removed that from the `xlsxwriter` engine.
 @jtratner ?
 Going to take a look tonight or tomorrow.
On Oct 7, 2013 7:36 PM, "jreback" notifications@github.com wrote:

> @jtratner https://github.com/jtratner ?
> 
> —
> Reply to this email directly or view it on GitHubhttps://github.com/pydata/pandas/pull/5122#issuecomment-25854366
> .
 @jtratner this ok for merging?
 Looks good. 
 Thanks for fixing this!
 And :+1: for not doing `_conv_value` where it's not needed.
