Added util/util.go and messages/message.go

`util.go` is meant to contain utility devices, like Vec3 and Brush structs
that may be useful for other sections of code. may eventually contain
angle helper function, brushreader, etc

`message.go` is meant to hold structures pertaining to .DEM file format
messages . These messages are the same as what the server and client use
to communicate with one another. Turns out what I was doing isn't
possible (which makes senses after I realized it), and so it's gonna get
redone. Probably with a m := Message(CONST).Init() or something, with
the Init setting metadata to hold extra info (ie sound info,
playerstatechange info, etc). Want to implement Message as an interface
somehow though, so that []Message can be used to store different types
of messages.