Add Main-Class in the manifest.

It's kinda useful.

Change-Id: Idc283d5ed3e96165f63708f02f7187779ea18399
Reviewed-on: http://gerrit.dmdirc.com/4061
Reviewed-by: Greg Holmes <greg@dmdirc.com>
Automatic-Compile: DMDirc Build Manager

build.gradle

@@ -121,6 +121,10 @@ jar {
     from { configurations.bundle.collect { it.isDirectory() ? it : zipTree(it) } } {
         exclude 'META-INF/**'
     }
+
+    manifest {
+        attributes 'Main-Class': 'com.dmdirc.Main'
+    }
 }
 
 buildscript {