Add PreLoginEvent

This PR adds `PreLoginEvent` from 1.7 BungeeCord to provide compatibility for new plugins. 
This implements #21 by @tf101-wizard.

**Note that these changes are untested** - I will pull this to master once I get confirmation that it works as intended for both protocols.
Artifacts for testing are located at http://ci.nowak-at.net/job/public~TUD_Bungee_PR/