Numeric columns returned as text

[tthibo]

When uploading a spreadsheet that contains numeric columns, the user receives a download
version that treats those columns as text.

To replicate: Upload the EPA example:
https://github.com/associatedpress/geomancer/blob/master/examples/Eqi_results_2013July22.xlsx

Select Median Household Income.

Check downloaded results. The EQI fields are now all showing number stored as text.

[evz]

Under the hood we are converting everything to CSV to make the internal plumbing a bit simpler and then just writing the file back out to the format that the person gave us originally. This would be a nice feature to consider eventually but I'm going to punt it for now just to reduce complexity for the short term.