add delayed job support to logweasel

because delayed job payloads don't support any kind of context separate
from the job method and args, we shove a hash onto the end of the args
list containing the logweasel id and then pop it back off before
invoking the job.
remove obsolete code
support DJ payloads other than PerformableMethod

if a payload doesn't support the `args` method (provided by
PerformableMethod) but does support `log_weasel_id` and `log_weasel_id=`
(provided by the new LogWeasel::DelayedJob::TransactionTrackablePayload)
then attach a transaction id to it.
fix plugin loading
arrays also respond to :[]
Merge pull request #2 from newcolabs/dj

add delayed job support to logweasel