Add login using session key

[DarkFighterLuke]

Hello, it would be great if a function to login using session key was written.

I tried to write it by myself, but I don't know how the API works. I will be very happy if someone could tell me the procedure to follow in order to understand the API behaviour and I will be happy to help you with this thing.

[DarkFighterLuke]

I managed to write a piece of code for session login (I didn't upload it on github yet See #36 for code). The authentication part seems to be fully working, but there is a problem: no master key is retrieved with this type of login, so no nodes decryption can be done.

I need help to get it work, especially because I have no knowledge of how the encryption/decryption of folders is done on Mega (in particular I can't figure out how to get the master key).