ECC.Encrypt is static

### Motivation:
[ECC.encrypt](https://github.com/yann2192/pyelliptic/blob/master/pyelliptic/ecc.py#L445) is a static method. As such, it is not affected by initialization parametres of the `ECC` class. In particular, it completely ignores the `curve` parametre and derives it from the [key](https://github.com/yann2192/pyelliptic/blob/master/pyelliptic/ecc.py#L450) that is used. The code should reflect this behaviour and not mislead the reader.

### Changelog:
* `node/connection.py :: CryptoPeerConnection.encrypt` changed to reflect the the static nature of `ECC.encrypt`.
* Tests added to document this behaviour.