Add support  of 'decimal' option to Series.to_csv and Dataframe.to_csv

closes #781
 
The 'decimal' option exists for read_csv method but not yet in 'to_csv' methods.
The lack of this option is particulary painful when we *have to* work with Excel with European regional settings. 

This modification add this option to both Series.to_csv and Dataframe.to_csv.
I don't understand you point.

If the float_format specified is something like '%.3f' just replacing the '.' by a ',' will lead to an uncorrect formater.