Implemented the "gmt" static property in the TimeZone+Zone extension.

Sources/Core/Extensions/TimeZone+Zone.swift

@@ -0,0 +1,10 @@
+import Foundation
+
+public extension TimeZone {
+    
+    // MARK: Zones
+
+    /// Greenwich Mean Time or UTC+0
+    static let gmt = TimeZone(secondsFromGMT: 0)
+    
+}