Add intercept and send raw data support (for unconnected data)

[mirasrael]

Also exposed few more host fields to API (for other programming languages).

Functions added:

• enet_host_get_received_data - adds access to received data buffer (useful for intercept hook)
• enet_host_send_raw, enet_host_send_raw_offset - allows to send raw data with host's socket
• enet_host_set_intercept - allows to set intercept callback (called for every received packet and gives a chance to add special packet handling, like processing unconnected data).
• enet_host_get_mtu - gets host's current MTU