Added functionality to delete files automatically on DROP FOREIGN TABLE command.
Also, fixed issues "cstore_fdw should error out on create if it cannot access the
file" and "cstore_fdw should return empty table if no data has been loaded".
Applied code review feedback.
Merge pull request #29 from samay-sharma/master

(1) delete files on DROP FOREIGN TABLE (2) don't error out for empty tables.

(1) Previously when user dropped the foreign table, the data and footer files weren't deleted and user needed to delete them manually. This wasn't convenient and sometimes it caused problems in situations where user recreated a table schema but with the same filename. This commit fixes the issue by automatically deleting the files when user issues DROP FOREIGN TABLE.

(2) Previously user couldn't run queries on a table where no data was copied in. This was because data and footer files were missing. In this commit these files are created when the table is created, so this issue is fixed. This change also has a useful side-effect: if the filename is not writable, user gets an error when trying to create the table.

patch by @samay-sharma
tested by @samay-sharma and @pykello,
cr by @pykello