Rename project as `pytezos-crypto`

The name of this repository, `aleph-pytezos`, indicates that there is something specific to aleph.im in this repository, while this is not the case.

I suggest to rename this project `pytezos-crypto` as to clear out this.

I also propose updating the README to be more clear on the focus of this fork:
https://github.com/aleph-im/aleph-pytezos/pull/12/files