(refs #204)Add some way to configure data directory.

1) system property of JVM (e.g. -Dgitbucket.home=PATH_TO_DATADIR)
2) java -jar gitbucket.war --data=PATH_TO_DATADIR
3) Add context parameter "gitbucket.home" to web.xml

src/main/java/JettyLauncher.java

@@ -27,6 +27,8 @@ public class JettyLauncher {
                         contextPath = dim[1];
                     } else if(dim[0].equals("--https") && (dim[1].equals("1") || dim[1].equals("true"))) {
                         forceHttps = true;
+                    } else if(dim[0].equals("--data")){
+                        System.setProperty("gitbucket.home", dim[1]);
                     }
                 }
             }