ignore rvm/rbenv droppings

I like gemsets, but that doesn't mean anyone else should...
add logging, in backwards compatible way
remove unneeded `if`
make invocation style consistent.
use double-quotes only when interpolating
mark as private
Fix wrong word/bad spelling in comment
add logging to failure backend as well
default to logging being on
rename the module
improve log messages based on code review and real use
module now has all logging code

Thisi makes our logging totally independent of whatever logging methods
are provided by Resque.  We use Resque.logger underneath, but the plugin
log statements now go through on interface that we control
ensure we make calls to Redis only once for logging

The log message in question needs to distinguish between "job is not
retriable" and "job IS retriable, but isn't actually being retried right
now".

We also don't want to go to Redis more than is required, so intermediate
variables to the rescue.  Also happens to straighten out that
double-negative, DeMorgan's Law-requiring `if` statement expression.
I can spell.  Really.  I can.
formatting
Merge remote-tracking branch 'github/master' into add-logging

Conflicts:
	lib/resque/plugins/retry.rb
Merge pull request #99 from stitchfix/add-logging

Add logging