Fix unhandled exception when a pull request isn't found

Without this patch the application blows up if a pull request is closed
and the card cannot be found on any of the boards.  This is because a
non nil value was being returned when a no return value should be
generated.
Add a due date to cards created

Without this patch the cards created by this app have no due date
associated with them.  This is a problem because I'd like to make sure
all cards have actions by 2 PM the business day following when they were
created.

This patch addresses the problem by checking if
TRELLO_SET_TARGET_RESPONSE_TIME has the value "true" and if so, sets a
due date on the newly created card.
Add timezone support and update README

Without this patch it is not clear how to get the application running in
Heroku to correctly set the due date of a created card to 2 PM for the
time zone the business is actually in.  This behavior is subject to the
TZ environmental variable which is set using `heroku config:add
TZ=America/Los_Angeles`

This patch addresses the problem by adding a configuration section to
the README which mentions the timezone configuration setting.

In addition, the patch also updates the README with information about
recently added features such as searching multiple boards to prevent
card duplication and archiving cards when a pull request is closed.
Merge branch 'add_due_date_to_new_cards'

* add_due_date_to_new_cards:
  Add timezone support and update README
  Add a due date to cards created
  Fix unhandled exception when a pull request isn't found