Implemented COPY <cstore_table> TO ... feature

COPY <cstore_table> TO ... statement is converted to COPY (select * from <cstore_table>) TO ... format and forwarded to native COPY function of postgresql

A minor bug in drop command is fixed to allow dropping of multiple cstore tables at once.
I think the name may be confused with ```COPY ... FROM ...``` statements, so what do you think about names like ```CopyOutCStoreTable```?