Add ipcidr protocol in reference with go-multiaddr

[lla-dane]

Tracks: multiformats/multiaddr#181
Adds the ipcidr protocol in py-multiaddr in reference with go-implementation

var TranscoderIPCIDR = NewTranscoderFromFunctions(ipcidrStB, ipcidrBtS, ipcidrValidate)

func ipcidrBtS(b []byte) (string, error) {
	if err := ipcidrValidate(b); err != nil {
		return "", err
	}
	return strconv.Itoa(int(b[0])), nil
}

func ipcidrStB(s string) ([]byte, error) {
	ipMask, err := strconv.ParseUint(s, 10, 8)
	if err != nil {
		return nil, err
	}
	return []byte{byte(uint8(ipMask))}, nil
}

func ipcidrValidate(b []byte) error {
	if len(b) != 1 {
		return fmt.Errorf("invalid length (should be == 1)")
	}
	return nil
}

@seetadev @acul71

[lla-dane]

Now will write the ipcidr + multiaddr tests as written in go implementation.

[seetadev]

@lla-dane : Great, thanks Abhinav.

Lets try and getting key protocols in the upcoming multiaddr release. We plan for another release 4 weeks from now.

[lla-dane]

@seetadev @acul71: I included the ipcidr type addresses in existing test cases, so now all the tests are running the same ipcidr address, as go-multiaddr.

[lla-dane]

@seetadev @acul71: I added the tests, I think this PR should be ready for merge.