New algorithms

Added support to

* Symmetric encryption: 3DES. http://www.w3.org/2001/04/xmlenc#tripledes-cbc 
* Asymmetric encryption: RSA-v1.5. http://www.w3.org/2001/04/xmlenc#rsa-1_5 

When making this support, I refactored the code to make adding new encryption methods even easier.

Removed dependencies to ursa since it's not working with latest node.js versions.