Curve25519 support

Implemented curve25519 support for dispersy, members can be constructed from a curve25519 key. This paves the way for the tunnelcommunity swtiching to curve25519 keys.
It uses
* libnacl, install using pip install libnacl
* libsodium, i used the pre-compiled dll file for windows from https://github.com/adamcaudill/libsodium-net/releases. Alternatively tweetnacl can be used.

Tested barebone signing/verifying using test_crypto, signing/verifying using test_member.