2.0.0-alpha.16: Merge pull request #208 from vapor/preparation-separate

New:

• Timestamps! (enabled by default on all entities)
• Entity is no longer required to conform to Preparation

To disable timestamps on a model:

public final class User: Entity {
    static let usesTimestamps = false
}

To disable db wide:

drop.database?.usesTimestamps = false

#208
#209