Bugfix: AspectJ installer extracts text files as US-ASCII

When extracting text or HTML files, special characters like German
umlauts "ÄÖÜäöüß" or copyright symbol "©" were destroyed while unpacking
the installer archive. As our files in Git SCM are all UTF-8, the
installer now also uses UTF-8 to read and write text files.

Fixes #270.

Signed-off-by: Alexander Kriegisch <Alexander@Kriegisch.name>

build/src/main/java/$installer$/org/aspectj/Main.java

 import javax.swing.border.CompoundBorder;
 import javax.swing.border.EmptyBorder;
 
+import static java.nio.charset.StandardCharsets.UTF_8;
+
 /**
  * Invoke the Installer gui. There are two ways to run without GUI by passing parameters to main:
  * <ol>
 	}
 
 	public static String stringFromStream(InputStream stream) throws IOException {
-		BufferedReader reader = new BufferedReader(new InputStreamReader(stream, "US-ASCII"));
+		BufferedReader reader = new BufferedReader(new InputStreamReader(stream, UTF_8));
 
 		StringBuilder ret = new StringBuilder();
 		int data;
 
 	public void writeTextStream(InputStream zis, File outputFile) throws IOException {
 		BufferedWriter os = new BufferedWriter(new FileWriter(outputFile));
-		BufferedReader r = new BufferedReader(new InputStreamReader(zis, "US-ASCII"));
+		BufferedReader r = new BufferedReader(new InputStreamReader(zis, UTF_8));
 
 		String l;
 		while ((l = r.readLine()) != null) {
 		}
 
 //		InputStream stream = url.openStream();
-//		BufferedReader reader = new BufferedReader(new InputStreamReader(stream, "US-ASCII"));
+//		BufferedReader reader = new BufferedReader(new InputStreamReader(stream, UTF_8));
 //		String line = reader.readLine();
 //		installPane.nBytes = Integer.parseInt(line);
 //
 
 	public void writeTextStream(ZipInputStream zis, File outputFile) throws IOException {
 		BufferedWriter os = new BufferedWriter(new FileWriter(outputFile));
-		BufferedReader r = new BufferedReader(new InputStreamReader(zis, "US-ASCII"));
+		BufferedReader r = new BufferedReader(new InputStreamReader(zis, UTF_8));
 
 		String l;
 		while ((l = r.readLine()) != null) {