Support for excel XLSX files

[slinkp]

There is a package for csv.dictreader-style access to old Excel files here:
ebdata/ebdata/parsing/excel.py
And it is used (largely untested) in the spreadsheet scraper
(ebdata/scrapers/general/spreadsheet/retrieval.py)
and in the admin UI import view
(obadmin/obadmin/admin/views.py).

We should add support for new-style xslx files. There's a python library for this:
http://www.lexicon.net/sjmachin/README.html
but it doesn't provide a DictReader-style interface, so one would have to be created.

[slinkp]

Ticket imported from Trac:
http://developer.openblockproject.org/ticket/277
Reported by: slinkp