[TACHYON-139] WebUI--Add the downloading file feature on the webUI

add download function to browse page
You don't need this method, already exists in the classpath: http://docs.guava-libraries.googlecode.com/git/javadoc/com/google/common/io/ByteStreams.html

If you need to limit the number of bytes copied, then you can do that with the InputStream by using something like a bounded input stream: http://commons.apache.org/proper/commons-io/apidocs/org/apache/commons/io/input/BoundedInputStream.html