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ARROW-218: Add optional API token authentication option to PR merge tool

You can use an API token with extremely limited privileges (i.e., only access to public GitHub repos), but this helps avoid rate limiting issues on shared outbound IP addresses.

Author: Wes McKinney <wesm@apache.org>

Closes #91 from wesm/ARROW-218 and squashes the following commits:

f45808c [Wes McKinney] Add optional GitHub API token to patch tool (to avoid rate limiting issues with unauthenticated requests)