update docs

[stevengj]

We decided that ruler.py was too generic a name, so I used imageruler as the name of the repo, and the program should be updated to match.

@mawc2019

[stevengj]

Partially addressed by #3. Mainly we just need updated docs and examples.

[ianwilliamson]

With the name change from ruler -> imageruler, the import in the notebook example is now broken.

[mawc2019]

I will update this in a new PR. Additionally, two minor issues on file organization are pending. Any suggestions?

[stevengj]

Would be good to have a readthedocs manual generated from the code docstrings. See https://docs.readthedocs.io/en/stable/tutorial/index.html