BUG : improved input clean up in Axes.{h|v}line

 - passing in a (N, 1) shaped array as input causes the zipping to
   generate an extra dimension
 - use squeeze to get rid of useless dimensions
 - use atleast_1d to make sure scalars remain 1D ((1, ) vs () shape)
 - closes #2197

@efiring I suspect there is a better way to do this.