add md5 function

[jeffbrennan]

addresses #21

I have a working implementation of an appended md5 column using the standard library hashlib.md5 function.

This likely won't work on larger tables as implementation includes a pylist collection and a list comprehension where the md5 hash is computed per-value before being appended to the pyarrow table.

Open to suggestions for improvement!

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One thing that is currently unhandled is columns containing null values. The binary_join_element_wise documentation has support for

1. emitting a null for the full concatenation (default)
2. skipping the null element
3. replacing the null element with a string

I'm in favor of either 2 or 3 - let me know your thoughts

[jeffbrennan]

@MrPowers