(maint) Move ENV testing into the base trello job spec

Without this patch the behavior of #save_settings is tested in the
higher level PullRequestJob spec files.  This is a problem because it
looks like there is no test coverage of the BaseTrelloJob class.

This patch moves the testing of #save_settings into the newly
established file for exercising behavior on the BaseTrelloJob class.
Add BaseTrelloJob#archive_card? instance method

Without this patch jobs that speak to trello don't have an easy way to
determine if they should archive the card or not.  This is a problem
because we'd like to make this behavior easy for end users to configure.

This patch addresses the problem by implementing the archive_card?
instance method which reads the ARCHIVE_CARD environment variable via
BaseTrelloJob#save_settings when the job is initially created and
before saving it to the database.
Only archive cards if ARCHIVE_CARD=true or yes

This patch finishes the work wiring up delayed jobs to the original
user-configured environment.  With this patch applied the
TrelloPullRequestClosedJob class will only archive a card if the
application environment was configured with ARCHIVE_CARD=true or
ARCHIVE_CARD=yes at the time the job was originally created.

The values are case insensitive.
Merge pull request #28 from jeffmccune/fix/master/dont_archive_cards

Only archive cards if ARCHIVE_CARD=true or yes