Don't automatically flush on every System.write().

It's handy because it ensures writes are flushed to the terminal before
any calls to read from stdin, but it's also gratuitously slow.

Instead, added a Stdout class with an explicit flush() method that can
be called by the user.

Fix #445.

doc/site/modules/io/stdout.markdown

@@ -0,0 +1,11 @@
+^title Stdout Class
+
+The standard output stream.
+
+## Static Methods
+
+### **flush()**
+
+Flushes all buffered data to the stream. Ensures any data written to stdout
+that is in the buffer gets written to the file or terminal that stdout is
+connected to.