enable use inside Jupyter notebooks

Tricky due to Jupyter's IO not supporting fileno/subprocess:
``` python
import cwltool.factory
f = cwltool.factory.Factory()
echo = f.make("v1.0/v1.0/echo-tool.cwl")
out = echo(in="foo")
```
```
File "/home/michael/src/jupyterenv/local/lib/python2.7/site-packages/cwltool/job.py", line 221, in run
cwd=self.outdir)
File "/usr/lib/python2.7/subprocess.py", line 703, in __init__
errread, errwrite), to_close = self._get_handles(stdin, stdout, stderr)
File "/usr/lib/python2.7/subprocess.py", line 1153, in _get_handles
errwrite = stderr.fileno()
File "/home/michael/src/jupyterenv/local/lib/python2.7/site-packages/ipykernel/iostream.py", line 307, in fileno
raise UnsupportedOperation("IOStream has no fileno.")
UnsupportedOperation: IOStream has no fileno.
```