inserted all tested changes again after reset to master

the merge conflict chaos was too hard ;-(

This is a continuation of:
https://github.com/pydata/pandas/pull/4404

Because:

* we hit aparently a GitHub bug
* the branches diverged too much.

I shall fix:

* in the excel.py there is a fix enabling reading xlsx files with both
datemodes: (see #4332)
* in the parsers.py there is the fix for readinh the header even if there
are additional rows (to be skipped) between a header and the data (see: #4340)
* a few doc imporvements