Inheritance support with delegation classes.

Hi

I already have made a pull request about this #1127, but as it was not merged or commented, I closed it, because it is getting a lot of changes over the pull.

Now, I open this issue to keep it documented the needing. If someone wants to suggest anything, is free for do it.

The idea is to make Table as Interface, then we could make something like this:

```
interface SoftDeleteTable {
val deleted: Column
}

fun ITable.withSoftDelete(): SoftDeleteTable = object: SoftDeleteTable, ITable by this {
// Anonymus implementation of SoftDelete that add columns to the current table.
override val deleted = bool("is_active").default(false)
}

open class CountryTable(val table: ITable = Table()): SoftDeleteTable by table.withSoftDelete() {
val name = varchar("name", 255)
val isActive = bool("is_active").default(false)
}

class Country: IntEntity {
object companion Table: CountryTable()

name by Table.name
isActive by Table.isActive
deleted by Table.deleted
}

```