Add script to summarize previous seven days on trac.buildbot.net

Copied from pull request 1257 from buildbot/buildbot.
Determine start and end days in main()
Fetch github pull requests
Add the fields that should be in the pull request summary

For now, sticking with the id, title, and url of the pull request.
Skip pull requests that were created before the start day

This could be a problem for documenting the pull requests that were
closed within the summary period.
Add each opened and closed pull requests
Use different time fields for opened vs closed pull requests

Check the created_at field for opened pull requests and the closed_at
field for closed pull requests. This way should get all pull requests
relevant for the summary period.
Create summary of Github pull requests
Per sa2ajj's comment, add in merged pull requests
Programmatically generate pull request summaries
Allow the first column to be ints
Use plain integers as the first column

Using integers makes the sorting much easier and simplifies the code
setting up the dict.
Use tablify_dict function
Tag the strings returned to reassemble in the right order
Format the summaries into something resembling a coherent message
Order the pull requests in ascending order
Merge seankelly/buildbot-infra:weekly-summary (PR #1)