Allow users to upload .xlsx instead of just .tsv

[adswafford]

It would be useful for users to be able to import .xlsx files into Qiita and then choose which sheet to read the metadata from when associating.

We can use:
https://bitbucket.org/openpyxl/openpyxl
or
http://pandas.pydata.org/pandas-docs/version/0.20/generated/pandas.read_excel.html if we have pandas in the environment already?

I put a Dec 2017 deadline as this will be needed for the V1 output from the Metadata Wizard.

[wasade]

Easiest is assuming information is only in the first sheet and to just disregard sheets altogether. A feature enhancement in the future could be to allow users to specify the sheet (or sheets) to use.

[adswafford]

That works for me.