add pandas and tests

[braaannigan]

Hi @itamarst - I extended the jsonencoder to handle pandas in my own code and it worked well. Wondering if you're interested in developing something like this as part of the main repo?

I've just done a quick pass on the code and tests to give you a sense of it. Basically, for a dataframe I have it output the shape, column names with dtypes and a sample of rows.

Let me know if you think it's worth pursuing. Could also just do an example in the readme of the main repo showing how to do this as a custom extension.
Liam