fixed #11 + some bigger refactorings

* fixed Qt sqldriver build instructions
* fixed HelperRepository
* added builder patterns to Table, Index and MigrationExecutionContext
* added override to everywhere
* simplified Base*CommandServices
* improved some names
* fixed some include guards
* switched to a custom type system, to enable more SQL types
* optimized structure extraction for postgresql
* tested with all visual studio compilers, Qt 4.8.5 and Qt 5.2.1
* gcc 4.8.0 has problems