Added helpers for getting status and for blocking wait until ready

Added a helper for writing sequential code. Usage:

~~~
source = BigML::Source.create(temp_file.path).wait_for_ready
dataset = source.to_dataset.wait_for_ready
model = dataset.to_model.wait_for_ready
~~~

Or chain them:

~~~
model = BigML::Source.create(temp_file.path).wait_for_ready.to_dataset.wait_for_ready.to_model.wait_for_ready
~~~