Saving workflow to a file

this is preliminary work to save a workflow to a file. a demo can be seen here:

http://nbviewer.ipython.org/urls/dl.dropbox.com/s/zjpzzxhdlst694u/savewf.ipynb

reasons:
- many workflows are getting complicated/hierarchical enough that people cannot edit or modify parameters easily in embedded workflows
- default workflows may change with versions, so saving the workflow in a file allows one to recreate the same specs used to run the workflow

Todo:

- [x] save config and node configs