Revert commits breaking API and binary OCPP 1.6 passwords

[robert-s-ubi]

These commits were based on a misunderstanding of the OCPP-J 1.6 specification, which clearly states that the password is a byte sequence and not a string. Also, they broke the ServerEvents#authenticateSession() API:

Revert "More password fixes."

This reverts commit fb5d0fb4c9e5cf8e1f14ab97ea5e15422fb441f4.

Revert "Fix password decoding."

This reverts commit cf20205e849a258ee5f4893a1d916ef8091f12d1.

Revert "Recommended by 1.6 spec is a 20 byte (40 chars) key."

This reverts commit f7b92a3d6ae51cce4e6bbdcc25148cb951e986d1.

This commit breaks the API, because the behaviour of the method is changed to return the last configuration instead of the default configuration:

Revert "A single instace, otherwise a static get() method makes no sense."

This reverts commit 953f50b3e7c4b975cae5ebcb74c2bd6e381d5e23.

[robert-s-ubi]

This PR reverts the repository back to the state as of commit 14d60bc1f29ff518ab94e4f682595b53391a565c (Update gson), which was the only valid commit in the series. The other ones only broke things.