Build a basic calendar view

Summary:
This is a very small step toward building a Status and possibly an Oncall tool.
Build a calendar view which renders months.

Much of my hesitance to bang these tools out is that dealing with
dates/calendaring is basically horrible, so I'm trying to ease into it.

This calendar is locale-aware and all that jazz.

Test Plan:

• See:

https://secure.phabricator.com/file/view/PHID-FILE-c07a9c663a7d040d2529/

• Verified that months have the right number of days, today is the right day

of the week, months begin on the day after previous months end on, etc.

Reviewed By: aran
Reviewers: jungejason, tuomaspelkonen, aran
Commenters: cwbeck, jungejason
CC: blair, aran, epriestley, cwbeck, jungejason
Differential Revision: 791