Open Document Format ODS support in read_excel()

This work should eventually lead to a reader and writer for ods spreadsheets (LibreOffice / OpenOffice Open Document Format). I am doing this work in the master of my clone of pandas. From the wiki I understood it is fine for me to commit to my master and rebase with upstream from time to time. Please let me know if I should follow another git workflow (this is my first PR).

This is what I have so far:
* It seems to make sense to implement an OdsFile and OdsWriter in pandas/io/excel.py
* I created ods test files identical to the other Excel test files.
* Basic reading works, but does not handle all corner cases correctly yet
* Tests and WritingOds are not implemented yet

I am not sure how to approach the following issues:

* For the tests, it would make sense to also use pandas/io/tests/test_excel.py (there would be a lot of code duplication otherwise). However, this would require some kind of a more elaborate test skipping mechanism than currently implemented. One could have ods dependencies installed, but not the Excel ones and vice versa. 
* When considering pandas could support more spreadsheet formats, does it make sense to rename pandas/io/excel.py to spreadsheet.py, and correspondingly, read_excel to read_spreadsheet? I guess this is not easy since it a lot of stuff out there already counts on the excel naming scheme.

For the record, there is one message on the [mailing list](https://groups.google.com/forum/?fromgroups#!topic/pydata/Zv-BXJNsP2U), and this PR will fix issue #2311.

ods support depends on [ezodf](https://github.com/T0ha/ezodf), and issue #2311 mentions some of the alternatives.