Fixed \Phalcon\Db classes

* Phalcon\Db\Adapter\Pdo\Postgresql
  - Implemented.

* Phalcon\Db\Adapter\Pdo\Sqlite
  - Implemented.

* Phalcon\Db\Adapter
  - Fixed describeIndexes method.
  - Fixed describeReferences method.

* Phalcon\Db\Adapter\Pdo
  - Added convertBoundParams method.

* Phalcon\Db\Adapter\Pdo\Mysql
  - Fixed describeColumns method.

* Phalcon\Db\Column
  - Fixed __set_state method.

* Phalcon\Db\Dialect\Postgresql
  - Fixed sql.

* Phalcon\Db\Dialect\sqlite
  - Deleted unused variable.

* Phalcon\Db\Index
  - Fixed __constructor method.

* Phalcon\Db\Reference
  - Fixed __set_state method.

* Unit tests that pass
  - DbBindTest.php
  - DbDialectTest.php
  - DbTest.php
  - DbDescribeTest.php