Direct converting numeric types to bool

Attached patch allow direct convertion of numeric types to bool like 
integer::bool.
Supported types:
  - smallint;
  - bigint;
  - real;
  - double precision;
  - decimal(numeric).

This functionality is helped with migration from Oracle.

-- 
Nikita Zhuchkov
Postgres Professional: http://www.postgrespro.com
The Russian Postgres Company