DbMigration supports altering UniqueConstraints and foreign keys (#1279)

* NEW: Ebean reads version from pop.properties and prints it at start up

* DDL generation and migration can append a header and platformMigration is prepared to append epilog & prolog to scripts

* FIX: Set identity only, if it is not the platform default type

* Updated reference scripts

* ADD basic support for foreign key migration

* DbMigration detects alter foreign keys

* tests

src/test/java/misc/migration/v1_2/DfkSetNull.java

@@ -0,0 +1,22 @@
+package misc.migration.v1_2;
+
+import io.ebean.annotation.ConstraintMode;
+import io.ebean.annotation.DbForeignKey;
+
+import javax.persistence.Entity;
+import javax.persistence.Id;
+import javax.persistence.ManyToOne;
+import javax.persistence.Table;
+
+@Entity
+@Table(name = "migtest_fk_set_null")
+public class DfkSetNull {
+
+  @Id
+  long id;
+
+  @ManyToOne
+  @DbForeignKey(onDelete = ConstraintMode.SET_NULL)
+  DfkOne one;
+
+}