BUG: parser can handle a common_format multi-column index (no row index cols), (GH4702)

closes #4702, should help with #5254

Will handle the format generated by ``to_csv`` (which has a an 'extra' line for the index names)
and the more 'common' format

```
In [4]:         data = """,a,a,a,b,c,c
   ...: ,q,r,s,t,u,v
   ...: one,1,2,3,4,5,6
   ...: two,7,8,9,10,11,12"""

In [6]: read_csv(StringIO(data),header=[0,1],index_col=0)
Out[6]: 
     a         b   c    
     q  r  s   t   u   v
one  1  2  3   4   5   6
two  7  8  9  10  11  12
```