Replace readable with observables

[SewerynKras]

Breaking changes:

• executor.execute is no longer an async function that sends the script and creates a Readable. Instead, it synchronously creates an observable that once subscribed to, sends the script and starts emitting it's results
• RemoteProcess (result of runAndStream) now uses Subjects instead of Readables to implement stdout and stderr
• Removed stream polyfill as it's no longer used

[grisha87]

As per remarks in the comments :)