Add convenience initializer for floating action button with single item

Example/Tests/JJFloatingActionButtonSpec.swift

@@ -455,5 +455,29 @@ class JJFloatingActionButtonSpec: QuickSpec {
                 expect(actionButton) == snapshot()
             }
         }
+
+        describe("JJFloatingActionButton using single item initializer") {
+            var actionButton: JJFloatingActionButton!
+            var action = "not done"
+
+            beforeEach {
+                actionButton = JJFloatingActionButton(image: #imageLiteral(resourceName: "Favourite"), action: { _ in
+                    action = "done!"
+                })
+            }
+
+            it("has one item") {
+                expect(actionButton.items.count) == 1
+            }
+
+            it("has the correct icon") {
+                expect(actionButton.items.first?.buttonImage) == #imageLiteral(resourceName: "Favourite")
+            }
+
+            it("performs action when tapped") {
+                actionButton.sendActions(for: .touchUpInside)
+                expect(action).toEventually(equal("done!"))
+            }
+        }
     }
 }

Sources/JJFloatingActionButton.swift

@@ -230,6 +230,20 @@ import UIKit
         setup()
     }
 
+    /// Initializes and returns a newly allocated floating action button object with the specified image and action.
+    ///
+    /// - Parameter image: The image of the action item. Default is `nil`.
+    /// - Parameter action: The action handler of the action item. Default is `nil`.
+    ///
+    /// - Returns: An initialized floating action button object.
+    ///
+    /// - SeeAlso: init(frame: CGRect)
+    ///
+    @objc public convenience init(image: UIImage, action: ((JJActionItem) -> Void)? = nil) {
+        self.init()
+        addItem(title: nil, image: image, action: action)
+    }
+
     internal lazy var itemContainerView: UIView = {
         let view = UIView()
         view.isUserInteractionEnabled = true