Support .mailmap parsing in mailmap_update.py

Also, AUTHORS/.mailmap/aboutus updated.  Before:
```
$ ./bin/mailmap_update.py 
[localhost] local: git log --format="%aN <%aE>" | sort -u

Read the text at the top of AUTHORS and the text at the top of
.mailmap for information on how to fix the below errors.  If someone
is missing from AUTHORS, add them where they would have been if they
were added after their first pull request was merged (checkout the
merge commit from the first pull request and see who is at the end of
the AUTHORS file at that commit.)

People who are in AUTHORS but not in git:

Jim Zhang <Hyriodula@gmail.com>
Patrick Poitras <acebulf@gmail.com>
Toon Verstraelen <Toon.Verstraelen@UGent.be>

People who are in git but not in AUTHORS:

Abhinav Chanda <abhinavchanda01@gmail.com>
Acebulf <acebulf@gmail.com>
Heiner Kirchhoffer <heiner.kirchhoffer@gmail.com>
Jim Zhang <hyriodula@gmail.com>
Soumya Dipta Biswas <sdb1323@gmail.com>
Sudhanshu Mishra <mrsud94@gmail.com>
Thomas Hisch <thomas.hisch@ims.co.at>
Toon Verstraelen <toon.verstraelen@ugent.be>
anurags92 <sharmaan@iitk.ac.in>
jcrist <crist042@umn.edu>
sushant <hiraysushant@gmail.com>

There are 310 people in git, and 302 (adjusted) people from AUTHORS

Fatal error: These two numbers are not the same!

Aborting.
```