Adding AES Walk Encryption support

This change is targetted specifically for storing repositories securely in
Amazon S3.
The basic code  has been implemented according to this stackoverflow
answer http://stackoverflow.com/q/992019 with few modifications.
AES cipher used : AES/CBC/PKCS5Padding
This code expects the user to set these properties in the file.
accesskey: <AWS_ACCESS_KEY>
secretkey: <AWS_SECRET_KEY>
acl: <public|private>
crypto.algorithm: <AES|PBEWithMD5AndDES>
password: <PASSWORD_FOR_ENCRYPTION>
crypto.keylength: <KEY_LENGTH> #For AES Encryption only. Default 128.

Note: For using strong encryptions Java Cryptography Extension (JCE)
Unlimited Strength Jurisdiction Policy Files needs to be installed
properly.

List of changes:
1. Adding class AesEncryption in WalkEncryption.java
   Constructor:
      * Create SecretKey given password and salt using PBKDF2WithHmacSHA1
   Encrypt:
      * Embedd IV in output stream to be stored along with the file.
      * Encrypt and store the rest of message
   Decrypt:
      * Extract IV and initialize the cipher
      * Decrypt the rest of the file
2. Modified AmazonS3.java to handle AES encryption

Signed-off-by: Srinath H S <srinath.hs@gmail.com>