Change how StepRange gets its 'unit' in creating an empty range

[quinnj]

Instead of relying on one(step) to create the empty range representation, get's the natural step of the StepRange through stop - start. The former behavior resulted in the following for date ranges:

julia> dr = Date(2014,1,1):Year(1):Date(2013,12,31) # empty range with start > stop and positive step
2014-01-01:1 year:2013-01-01

cc: @JeffBezanson
This doesn't need to be 0.3, but I don't know that it hurts anything if we want to get it out of the way.

[StefanKarpinski]

This kind of seems like a bug fix to me, in which case it makes sense to merge for 0.3.

[quinnj]

Thanks!