Create MBR partitions

[bestouff]

There's a way to write an MBR, but no way to add some partitions in it.

[Kilobyte22]

This is closely related to #1 and will be fixed with it. If i can find time this week, this might even happen before weekend. Don't count on it though

[r-darwish]

Shouldn't exposing MBR::partitions as & mut be sufficient? The user will modify MBR::partitions and then call MBR::write_mbr