Fixed transporting task with the OGDS Transporter between different admin_units.

The commits 9b75a9d and 910c829 brakes transporting tasks with the
`opengever.ogds.base.transporter.Transporter` from one admin unit to another.

I've added an tests for transporting tasks and refactored the `Transporter` object creation completely.
Dropped no longer used interface `ogds.base.interfaces.IObjectCreator`.
Dropped the ITransporter utility and replace it with a simple object.
Merge pull request #693 from 4teamwork/pg_ogds_transporter_fix

Fixed transporting task with the OGDS Transporter between different admin_units.
Fixed MigrateAdminUnitOrgUnitSchema upgradestep, get foreign key constraints names dynamically.
Fixed MigrateAdminUnitOrgUnitSchema upgradestep: dropped unnecessary column change.
Fixed IncreaseColumnLength upgradestep, get foreign key constraints names dynamically.
Fixes IncreaseColumnLength upgradestep for oracle, removed wrong `nullable` parameter.
Dropped not necessary `new_column` attribute in IncreaseColumnLength upgradestep.
Fixed MigrateTaskTable upgradestep.

Migrate data before making the `issuing_orgunit` column required,
because the oracle DB is more strict in this case.
Disable temporarily removing unique constraints when using Oracle as OGDS DB.

Because oracle can handle column renames with unique constraints.
Fixed DB connection setup in the SchemaMigration class.

This allows transaction rollbacks when the upgrade fails.
Made AlterSequenceNumberType upgradestep oracle conform.

Because oracle does not support changing column type's from String to Integer,
we had to implement an oracle specific way with an temporary column.