Update improvements

Include NzbDrone.Update in mono/osx package
Do not ignore certificate warnings for services
Check hash before extracting update
New: Update support for Linux/OS X - see the wiki for more information

src/NzbDrone.Core/Update/UpdateMechanism.cs

@@ -0,0 +1,8 @@
+namespace NzbDrone.Core.Update
+{
+    public enum UpdateMechanism
+    {
+        BuiltIn = 0,
+        Script = 1
+    }
+}