Use the new dag api and use ipld links for both heads and next pointers

This change is backwards compatible, meaning the old logs can still be read.

Also updated IPFS to latest version as the current installed one has breaking changes to the dag api.
This way, we are already using the new dag api.

API.md

@@ -65,7 +65,7 @@ const tails = log.tails
 
 #### append(data)
 
-Append an entry to the log. Returns a *Promise* that resolves to the updated `Log`.
+Append an entry to the log. Returns a *Promise* that resolves to the added `Entry`.
 
 `ipfs` IPFS instance.
 
@@ -75,31 +75,31 @@ Append an entry to the log. Returns a *Promise* that resolves to the updated `Lo
 
 ```javascript
 log.append({ some: 'data' })
-  .then(log => log.append('text'))
-  .then(log => console.log(log.values))
+  .then(() => log.append('text'))
+  .then(() => console.log(log.values))
 
-// [ 
-//   { 
-//     hash: 'QmV1KFxZnaguPFp57PMXz5Dd1tf6z9U8csJQoy4xWDzzts',
+// [
+//   {
+//     hash: 'zdpuAkYarxnBQD7dQcr5onfzCTkTW8FebzC9nY5FviAveSzCL',
 //     id: 'A',
 //     payload: { some: 'data' },
 //     next: [],
-//     v: 0,
-//     clock: LamportClock { id: 'A', time: 0 } 
+//     v: 1,
+//     clock: LamportClock { id: 'A', time: 0 }
 //   },
-//   { hash: 'QmSxe4Shd7jt4ExyoBjtvgi1UabNKrZfRJKptwUmSa843u',
+//   { hash: 'zdpuAx4RKTtSkrpA8LKP5vzUdAoWExxLHrRjK2s8YEayDBQDC',
 //     id: 'A',
 //     payload: 'text',
-//     next: [ 'QmV1KFxZnaguPFp57PMXz5Dd1tf6z9U8csJQoy4xWDzzts' ],
-//     v: 0,
-//     clock: LamportClock { id: 'A', time: 1 } 
-//   } 
+//     next: [ 'zdpuAkYarxnBQD7dQcr5onfzCTkTW8FebzC9nY5FviAveSzCL' ],
+//     v: 1,
+//     clock: LamportClock { id: 'A', time: 1 }
+//   }
 // ]
 ```
 
 #### join(log, [length], [id])
 
-Join the log with another log. Returns a Promise that resolves to a `Log` instance. The size of the joined log can be specified by giving `length` argument. 
+Join the log with another log. Returns a Promise that resolves to a `Log` instance. The size of the joined log can be specified by giving `length` argument.
 
 ```javascript
 // log1.values ==> ['A', 'B', 'C']
@@ -121,6 +121,8 @@ log1.toMultihash()
 // QmSUrxz12UDsuuQMjzBQ4NDGyYprhFJbQefgeRiomQ5j6T
 ```
 
+You can take the hash and inspect the DAG in [explore.ipld.io](https://explore.ipld.io/).
+
 ### toBuffer()
 
 Converts the log to a `Buffer` that contains the log as JSON.stringified `string`. Returns a `Buffer`.
@@ -165,6 +167,8 @@ Returns the multihash of the log.
 
 Converting the log to a multihash will persist the log to IPFS, thus causing side effects.
 
+You can take the hash and inspect the DAG in [explore.ipld.io](https://explore.ipld.io/).
+
 #### Log.fromMultihash(ipfs, multihash, [length=-1])
 
 Create a `Log` from a multihash.