Usage of nested fields

[cardin]

I've opened a pull request #17 for the usage of nested fields.

Currently es-dedupe - unintentionally - is able to receive nested fields as a timestamp. E.g. (--timestamp myfield.isotimestamp). However, attempting to use a nested field as a UID will result in KeyError in Python.

What I've done is to include python-benedict, which allows users to provide nested fields (e.g. location.postal) as a valid field, as python-benedict can use dot notation to reference a nested key.