Expose Buffer on ipfs objects

This lets you construct a Buffer even in browser code that doesn't have access to the Node Buffer global, so that you can pass it back to IPFS API calls that want a Buffer.

Still need to do the documentation.

Author: interfect

src/core/index.js

@@ -66,4 +66,9 @@ function IPFS (repoInstance) {
   this.files = files(this)
   this.bitswap = bitswap(this)
   this.ping = ping(this)
+
+  // Export Buffer to somewhere where API consumers can get at it, for when
+  //  we're running in a browser but still expect Buffer instances to be passed
+  //  in.
+  this.Buffer = Buffer
 }