Add ability to upload excel files

Needs some refactoring.
Make PEP-8 happy
Remove erroneously added file
Add date parsing for excel files

Date parsing from excel files has the potential to be error prone. For more information see http://www.lexicon.net/sjmachin/xlrd.html

@YolinaS
@robyoung
Verify XLS files can be parsed
Raise a parse error if cell errors are found

As all the data is being loaded directly into backdrop it seems better to fail early.
PEP8
Use example for permissions.

Remove my email but still make it obvious what the format should be.
Get rid of magic numbers
Add newline
Merge pull request #122 from alphagov/accept-excel-files

Accept excel files