DataFrame only containing one column named Index breaks the constructor

[chiraganand]

!julia> TSFrame(DataFrame(Index=[1, 2]))
 ERROR: ArgumentError: Column Index is already present in the data frame which is not allowed when `makeunique=true`

This call should actually output an empty TSFrame with 0 columns but with the provided index.

Ref #183

[Prajna1999]

taking this up @chiraganand CC @ayushpatnaikgit