Feature issue#12

Shards backed by foreign tables marked correctly in the metadata.

Tested with 

1) Postgres 9.4
    - foreign table - select/insert
    - normal table (RELKIND_RELATION)  - select/insert

2) Citus 3.0.1
    - foreign table - select problematic - send mail to @jasonmp85 . But I don't think it is related to this fix, because when I test with undoing the fix, we still have the problem.
    - foreign table - insert OK
    - normal table (RELKIND_RELATION)  - select/insert


closes #12