ENH: Enable ExcelWriter to construct in-memory sheets
Add support for StringIO/BytesIO to ExcelWriter
Add vbench support for writing excel files
Add support for serializing lists/dicts to strings
Fix bug when reading blank excel sheets

closes #8188
closes #7074
closes #6403
closes #7171

@jreback The vbench for openpyxl is very slow ~ around 300s on my computer. This is around 10x slower than the next slowest test (sql).  Should I reduce the df size for this test? 
 yes, a vbench should be < 1s if at all possible, so reduce the size on these.
 I think it is finished once green - I snuck in one more Excel-related issue (testing xlwt on Py34).

Openpyxl is very slow compared to xlsxwriter (5x slower)
 ahh, the `xlwt` just got ported to py3. finally! excellent.

yes, `openpyxl` does have this perf issue (and why `xlsxwriter` is the default). Though the author is working on making it much better.

ping when green.
 @jreback ready
 minor doc comment. ping after you have pushed for merging.
 @jreback Added those.
 Rebased
 @bashtage thanks for this!
