BUG: wrong index name during read_csv if using usecols
Closes #4201

If user passes usecols and not names, then ensure that the
inferred names refer to the used columns, not the document's
columns.

@guyrt thanks again!
 another issue for you! https://github.com/pydata/pandas/issues/4363
 and one more! https://github.com/pydata/pandas/issues/4880
 @guyrt any luck with either of these?
 haven't looked at them yet - working on 4335
