0.2.0

• Compatible with ENet 1.3.18
• Refine some trait requirements and derives
• Reduce allocations introduced by Rust port (#1)
• Reduce enet_malloc overhead
• Adjust Socket::receive interface to one which takes a pre-allocated buffer
• Add #![no_std] support (by disabling std feature)
• Add MTU_MAX constant (an alias of ENET_PROTOCOL_MAXIMUM_MTU)
• Add functions:
  • Host::mtu
  • Host::set_mtu
  • Peer::mtu
  • Peer::set_mtu
• Remove redundant ENET_ prefix on consts