Add convenience capability to initialize a dag directly from a dict. …

[gregnordin]

…Add corresponding test

I saw that add the pull request "add dict default arg to dag constructor #24" was never finished so I went ahead and did what you requested:

1. Modified init() to optionally take a dict of dicts to initialize dag
2. Updated the docstring
3. Added a test

I think it should be all ready to accept from what I understand needed to be done.

[gregnordin]

Oh, and also--thanks for writing this package! I'm finding it to be quite useful.