Moved off of reader.Peek so that the reader can advance
Moved off of reader.Peek so that the reader can advance

* got rid of `.Peek` for `.Read` which will advance the a certain
  number of bytes. I don't like the syntax as much but its the
  easiest way to get the required data
* resized the header as it seemed to be off by 3 bytes
* actually saving the demo file into the buffer

gophergala/csgo_parser,mattcan/csgodem,mattcan/csgodem,gophergala/csgo_parser