Add access-token/cipher-aes-256-gcm module

Add module for AES-256-GCM encryption
Add module dependency to package.json
Add mocha tests
Add access-token sources to coffee sources
Remove key derivation logic from module

Module aes-256-gcm should receive a buffer as the key
Add KEY_SIZE as export
Refactor tests to reflect change
Add tests for key validation
Rename required module in test
Add AccessTokenManager

Add AccessTokenManager as main module for access-token
Add module dependency to package.json
Add mocha tests
Merge branch 'access-token' into develop