Enable updating to/from non `%ProgramFiles%` locations

Closes #13 (closed) and adds "Portable/adminless" installations

java/net/i2p/router/Elevator.java

+package net.i2p.router;
+
+import com.sun.jna.WString;
+import com.sun.jna.platform.win32.Kernel32;
+import com.sun.jna.platform.win32.Kernel32Util;
+
+public class Elevator {
+    public static void main(String... args) {
+        executeAsAdministrator("c:\\windows\\system32\\notepad.exe", "");
+    }
+
+    public static void executeAsAdministrator(String command, String args) {
+        if (command == "" || command == null) {
+            System.out.println("No command specified");
+            return;
+        }
+        Shell32X.SHELLEXECUTEINFO execInfo = new Shell32X.SHELLEXECUTEINFO();
+        execInfo.lpFile = new WString(command);
+        if (args != null)
+            execInfo.lpParameters = new WString(args);
+        execInfo.nShow = Shell32X.SW_SHOWDEFAULT;
+        execInfo.fMask = Shell32X.SEE_MASK_NOCLOSEPROCESS;
+        execInfo.lpVerb = new WString("runas");
+        boolean result = Shell32X.INSTANCE.ShellExecuteEx(execInfo);
+
+        if (!result) {
+            int lastError = Kernel32.INSTANCE.GetLastError();
+            String errorMessage = Kernel32Util.formatMessageFromLastErrorCode(lastError);
+            throw new RuntimeException("Error performing elevation: " + lastError + ": " + errorMessage + " (apperror="
+                    + execInfo.hInstApp + ")");
+        }
+    }
+}