Add type/value aliases for scalaz.\/ and co.

Add type/value aliases for:
* Disjunction  <=> \/
* DLeft        <=> -\/
* DRight       <=> \/-
* DisjunctionT <=> EitherT

(cherry picked from commit 32cd731)

Author: Adelbert Chang

core/src/main/scala/scalaz/package.scala

@@ -299,5 +299,16 @@ package object scalaz {
   val IMap = ==>>
 
   type GlorifiedTuple[+A, +B] = A \/ B
-  type Disjunction[+A, +B] = A \/ B
+
+  type Disjunction[+A, +B] = \/[A, B]
+  val Disjunction = \/
+
+  type DLeft[+A] = -\/[A]
+  val DLeft = -\/
+
+  type DRight[+B] = \/-[B]
+  val DRight = \/-
+
+  type DisjunctionT[F[_], A, B] = EitherT[F, A, B]
+  val DisjunctionT = EitherT
 }