Cleanup database schema

* Change autoincrement from ````1```` to ````true````
* Remove unnecessary default values
* Modify notnull values

See #14
Workaround: not-null w/o default

It is not possible to specify a database field as not-null and without
any default value. To workaround this issue we explicitly specify a
working default value for the ```modified``` DB field.

See owncloud/core#5819
Merge pull request #19 from owncloud/db_schema_cleanup

Db schema cleanup