Added functionality to use foreignSchema attribute of foreign-key tag…

[surekap]

Reference: https://github.com/propelorm/Propel/issues/191

The foreignSchema attribute of the foreign-key tag can be used on MySQL to link to other databases According to the documentation provided at: http://propelorm.org/documentation/cookbook/using-sql-schemas.html

In databases which support schemas (MySQL), there is no way to add foreign keys across databases.

This PR enables the foreign key relationships by using the database name as the schema name in these cases.

Tested with MySQL

[dereuromark]

ping @surekap