index_as_columns option in ctable.fromdataframe()

[rytsim]

Improves on #344

Makes df -->ct-->df rountrip possible.

• Added an option index_as_columns to save the index of a dataframe in ctable.fromdataframe() method.
• Changed the ctable.todataframe() method to check for dataframe_index_columns attribute and if found set the dataframe index to this column before returning it.

[coveralls]

Coverage decreased (-0.08%) to 65.303% when pulling cf62f31d3fc7754f6cfa56e72630ba1317aaf145 on rytsim:master into 8df64b4c228ffd2ffcbdea94b393e26578f4c8f9 on Blosc:master.

[FrancescAlted]

Looks good to me; I have added a couple of comments though. Also, a couple of test units would be needed.

Thanks!