Use the descendents_tracker gem

The gem handles threading and some other edge cases. It's a small gem so
it makes sense to use it rather than reimplement.
Merge pull request #9 from 25-ventures/refactor/descendant_tracker

Use the descendents_tracker gem
Allow indifferent access for the data hash

This only lets the data has be accessed via [] with both string and
symbol keys. Other methods to access the data may not work.
Merge pull request #10 from 25-ventures/feature/data-indifferent-access

Allow indifferent access for the data hash
keyword arguments are a little silly for single params
Update Pipedrive to use the latest interface

Pipedrive makes no distinction between leads and contacts.