Crypto support:

Implements optional encryption support for redis session store. According to the [OWASP Session management recommendations](https://www.owasp.org/index.php/Session_Management_Cheat_Sheet#Session_ID_Content_.28or_Value.29) when working with sensitive information stored in server side sessions they should be encrypted.

Public options:
 secret (String): Optional passphrase for symmetric encryption support.
 algorithm (String): Optional algorithm to use (Default aes-256-ctr).

Private functions:
 encryptData(<String>plaintext): Wrapper that handles encryption & digest creation
 decryptData(<String>ciphertext): Wrapper that handles verification of HMAC & decryption
 digest(<String>key, <String>ciphertext): Handles HMAC creation
 encrypt(<String>key, <String>pt, <String>algo): Provides encryption from crypto module
 decrypt(<String>key, <String>ct, <String>algo): Provides decryption from crypto module.

Test:
 Added separate test file.

Readme.md
 Added doc for new option