Add support for scripting schemas.

Added Schema class to the model.
During "Script" operation, writes new "schemas.sql" script that creates DB schemas.
When scripting objects to SQL files, all files are named as "[{schema}].[{object_name}].sql" to avoid conflicts.
This resolves the previously reported [issue](https://github.com/sethreno/schemazen/issues/20) with having the same table name in two different schemas (e.g. app.batch and import.batch).