Display usage when no args.

src/main/kotlin/com/beust/kobalt/Args.kt

@@ -7,7 +7,7 @@ class Args {
     var targets: List<String> = arrayListOf()
 
     @Parameter(names = arrayOf("-bf", "--buildFile"), description = "The build file")
-    var buildFile: String? = null
+    var buildFile: String = "Build.kt"
 
     @Parameter(names = arrayOf("--checkVersions"), description = "Check if there are any newer versions of the " +
             "dependencies")
@@ -17,6 +17,9 @@ class Args {
             "actually running them")
     var dryRun: Boolean = false
 
+    @Parameter(names = arrayOf("--help", "--usage"), description = "Display the help")
+    var usage: Boolean = false
+
     @Parameter(names = arrayOf("-i", "--init"), description = "Create a new build file based on the current project")
     var init: Boolean = false
 

src/main/kotlin/com/beust/kobalt/Main.kt

@@ -109,6 +109,8 @@ private class Main @Inject constructor(
             //
             Wrapper().install()
             ProjectGenerator().run(args)
+        } else if (args.usage || args.targets.isEmpty()) {
+            jc.usage()
         } else {
             if (! buildFile.exists()) {
                 jc.usage()