Don't import `fmt`

[johanbrandhorst]

The only reason to import fmt appears to be because of the following line of code:

return fmt.Sprintf("CloseEvent: (%s) (%d) %s", cleanStmt, e.Code, e.Reason)

This could trivially be replaced by

return "CloseEvent: (" + cleanStmt + ") (" + strconv.Itoa(e.Code) + ") " + e.Reason

This would lead to smaller file sizes for the projects using the websocket bindings.

[johanbrandhorst]

See explanation in #19. Closing this.