excel reader & skip row between data & header & docs
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 improvements
