Adding custom support for boolean casting based on connection

The default PHP boolean-to-string casting does not work for some
database connection types (e.g. PostgreSQL).  Added a BOOLEAN
type to the Column class, and then the cast() method delegates
to the a new boolean_to_string() method in the Connection class.

By default the boolean_to_string() method returns the standard
PHP string cast for the boolean, retaining the original behavior.
This method can be overridden in adapter classes to provides custom
casting of boolean values.

Added an overridden boolean_to_string() method into the
adapters/PgsqlAdapter.php class which casts the boolean values
false and true to the strings "0" and "1" respectively.  These
string values are then recognized by the Postgres database as
boolean values, as opposed to the values "" and "1" generated
by the standard PHP cast.
Updated ColumnTest for new BOOLEAN type cast
fixed indentation formatting to match existing code
Updated tests for boolean value casting

Updated the ColumnTest::assert_cast() to test that boolean values are
being casted using the Connection boolean_to_string() method.

Added a PostgreSQL specific test to ensure that boolean values are
being casted as the correct string value.
Updated PgsqlAdapter boolean_to_string() for Postgres boolean values

Modified the boolean_to_string() method in the PgsqlAdapter class to
properly convert values that PHP considers a boolean, as well as
what PostgreSQL considers a boolean value.

E.g., PostgreSQL considers the values 'f', 'false', 'n', 'no', and
'off' as valid values for `false`, but PHP does not.  The original
implementation of this method would return `true` for one of these
values which is incorrect for a PostgreSQL database.

Also modified the base Connection class boolean_to_string() method
to first cast its `$value` parameter as a boolean before casting
it to a string.  This will account for PHP's loose type-casting
and make sure the method returns a consistent casted value.
Shortened the false condition in PgsqlAdapter::boolean_to_string()

While the previous version was explicit and descriptive, it wasn't
that elegant.  This is much nicer.
Merge pull request #423 from jpfuentes2/getloaded-add_boolean_value_cast

Getloaded add boolean value cast