Use WebCrypto instead of SJCL Library -> better performance

[klues]

AsTeRICS is currently using the SJCL library for encryption and decryption.

This benchmark shows that the WebCrypto API achieves much better performance since it can directly use the browsers API and hardware acceleration.

Goal: replace SJCL library by WebCrypto API

Notes and Requirements:

• the transistion between SJCL and WebCrypto has to be seamlessly, so some sort of conversion of the SJCL storage format and WebCrypto has to be implemented.
• Assessment of Web Crypto capabilities of IE - is it's implementation good enough? see caniuse.com
• Do many unit tests before implementing this!

[alexangelov14]

Hello @klues , whenever possible, could you please review PR #667 and give me feedback?