Store lines in to separate files

Hi

I have files contain timestamp. I'd like to parse row by row and put
them into file by timestamp.
in example

original file:
20140801,...,...,...,...,...
20140802,...,...,...,...,...
20140801,...,...,...,...,...
...

So I'd like to parse this rows to separate files 20140801 and 20140802
so that file
20140801.csv contains:
20140801,...,...,...,...,...
20140801,...,...,...,...,...

and 20140802.csv contains
20140802,...,...,...,...,...

I tried to write my own custom StorageFunc but as much I understand I
can not do it there.
I read about MultyStorage maybe this is the right tool to try? Or Pig
totally wrong tool for that problem?