Move commandline handling to Java

The gnirehtet commands were exposed through bash ("gnirehtet", for Linux
and Mac OS) and batch ("gnirehtet.cmd", for Windows) scripts.

Move the handling to the Java code, so the behavior is consistent across
both platforms.

Consequently, rename relay.jar to gnirehtet.jar.

README.md

@@ -45,7 +45,7 @@ Download the [latest release][latest]:
 Then extract it. You get three files:
  - `gnirehtet`
  - `gnirehtet.apk`
- - `relay.jar`
+ - `gnirehtet.jar`
 
 
 ## Run (simple)
@@ -97,12 +97,6 @@ To stop a client:
 
     ./gnirehtet stop [serial]
 
-On Windows, the server is started in a separate window. To stop it, either close
-the window or kill the server from anywhere:
-
-    gnirehtet killserver     # stop the relay server only
-    gnirehtet kill [serial]  # also stop the client
-
 The _serial_ parameter is required only if `adb devices` outputs more than one
 device.
 
@@ -111,14 +105,12 @@ For advanced options, call `./gnirehtet` without arguments to get more details.
 
 ## Run manually
 
-The `gnirehtet` script just exposes an interface for calling simple commands.
-You can call them manually.
-
-_Note: On Windows, replace `adb` by `adb.exe`._
+The `gnirehtet` program exposes a simple command-line interface that executes
+lower-level commands. You can call them manually instead.
 
 To start the relay server:
 
-    java -jar relay.jar
+    java -jar gnirehtet.jar relay
 
 To install the apk: