add authentication with public key

This pull request contains:
1. I have assign the base name of jar package as "petuum". The previous one was following the name of top directory.
2. I have fixed the error in README_java.txt
3. I have add support for authentication with public key. The demo is in Test.java. 
One more thing needs to be noted that the package I used for SSH access only supports DSA and RSA key.