adding new Is* methods that can be used to check log level to ensure log args aren't evaluated in certain conditions

This methods will be useful so that more involved logging can be turned on when the code is in a lower debug state

i.e.  
JSON serialized Request and Response objects when in debugging mode, which won't be logged when in production.

if log.IsTrace() {
  b, _ := json.MarshalIdent(struct), "", "  ")
  log.Trace(string(b))
}