Implement new BZZ APIs for browser

[PaulLeCam]

Ideally matching the node ones as much as possible, but supporting browser APIs such as Blob, FileReader, etc.

[AuHau]

Yeah, this will be quiet important. Especially not being able to download directory in browser is quite problematic.

Upon some research, there is support for streams in a browser using readable-stream. I will try to make an initial PR that will implement this package for using downloadObservable and downloadDirectory for browser as well.

[AuHau]

Do you think there is still some more API to be implemented in browsers? I think the only "node" specific APIs are those related to FS, which won't be ever implemented browser...

[PaulLeCam]

I was mostly thinking about supporting APIs that are available in the browser but AFAIK not in node, such as Blob, File, FileList and ReadableStream.

For example we could have a downloadBlob() function, uploadFile() could accept File and ReadableStream as inputs and uploadDirectory() could accept a FileList input, that would make it very simple to upload the contents of an <input type="file"> to Swarm.
Basically the same way the node-specific APIs make it easy to interact with the local FS, I think browser-specific APIs could be used to support higher-level application use cases.