[Removed] umsg stuff from net library

Closes #2
[Add] methods to net library

I left out Read-/WriteType, because it can be used to get sensitive types.
I am not sure about Vector and Angle, they can be sent as floats/doubles, after all. The wiki says it actually compressed them, though.
[Fixed] net message library not loading

So apparently I forgot one "end". I could swear it worked fine when I tested it...
I probably added bytesWritten without testing it, because it doesn't even work for me...
[Fixed] Slight enrror in __index metamethod for polys was using nil index value.
[Added] net.canSend function
[Fix] screen code not being sent properly

Sometimes screens were not loaded on the client and did not receive their code properly
[Fix] being unable to update screens

Also, CodeSent error still occasionally appears. Needs further debugging
[Fix] in timer library

timer.simple was throwing an error if the entity got removed while the timer was running

Fixes #53
[Change] the check in timer.lua to IsValid

As suggested by Divran in 930a1250e0e1bf7fc1a824004e99d86ce0ab252f
[Fix] some netlibs not doing typechecks