Add .xlsx export method

[devinbalkind]

Minimally we should catch cases where this fails & report nicely to the user.
Better solutions are:
(1) Use .xlsx instead (requries using http://xlsxwriter.readthedocs.org/en/latest/index.html instead of xlwt)
(2) Export a multi-sheet workbook
Ideally all 3 are done:
.xls requests give a multi-sheet workbook with a user-warning
.xlsx requests are possible
The simple warning is the 'easy' part
Multi-sheet workbook seems fairly simple.
xlsx export is not really 'hard' but not 'easy'

[devinbalkind]

anurag029 said:

I have tried to implement the multi-sheet workbook export using xlwt - anurag-ks@926c0d3
Need a review on it.

[nursix]

Anaurag's multi-sheet export is now in trunk, which prevents the crash. The implementation of the xslx export options is still desirable, however - even though such massive exports are rather rare (and super-size spreadsheets of questionable value).

Downgrading to minor enhancement.