fix: remove invalid multicodec2string mappings (#137)

* refactor: remove Codecs table
* chore: go-cid 0.2.0

Codec table was missing dag-json and it had invalid code for dag-cbor.
It also had invalid string representation of dag-pb -- it was using
'protobuf' which is a totally different code.

This removes invalid mappings entirely.
https://github.com/multiformats/go-multicodec should be used instead.

README.md

@@ -59,10 +59,17 @@ fmt.Println("Got CID: ", c)
 #### Creating a CID from scratch
 
 ```go
+
+import (
+  cid "github.com/ipfs/go-cid"
+  mc "github.com/multiformats/go-multicodec"
+  mh "github.com/multiformats/go-multihash"
+)
+
 // Create a cid manually by specifying the 'prefix' parameters
 pref := cid.Prefix{
 	Version: 1,
-	Codec: cid.Raw,
+	Codec: mc.Raw,
 	MhType: mh.SHA2_256,
 	MhLength: -1, // default length
 }