add pg inherits support

This patch is adding `postgres` inherits support to `slick`.

Sample codes for postgres inherits are as below:
```scala
/// 0. common columns
abstract class BaseT[T](tag: Tag, tname: String = "test_tab1") extends Table[T](tag, tname) {
  def col1 = column[String]("COL1")
  def col2 = column[String]("COL2")
  def col3 = column[String]("COL3")
  def col4 = column[Int]("COL4")
}

/// 1. base table
case class Tab(col1: String, col2: String, col3: String, col4: Int)
 class Tabs(tag: Tag) extends BaseT[Tab](tag, "test_tab1") {
  def * = (col1, col2, col3, col4) <> (Tab.tupled, Tab.unapply)
}
val tabs = TableQuery[Tabs]

/// 2. inheriting table
case class Tab1(col1: String, col2: String, col3: String, col4: Int, col5: Long)
class Tabs1(tag: Tag) extends BaseT[Tab1](tag, "test_tab2") with InheritingTable {
  val inherited = tabs.baseTableRow
  def col5 = column[Long]("col5")

  def * = (col1, col2, col3, col4, col5) <> (Tab1.tupled, Tab1.unapply)
}
val tabs1 = TableQuery[Tabs1]

/// 3. using it
val q1 = tabs1.sortBy(_.col4)
```

Pls help review and give your suggestions.