Issue #118: Added a bypass handler for authorisation

Fixes #118 

@rcrichton a quick one if you could please review? Basically on rerun we bypass authorization, so the ctx isn't setup with the authorized channel. Added a bypass handler that sets this up using the channel that the original transaction ran on.

Completed with thanks to @BMartinos 
Pretty minor comment, but this can be done as follows: `Transaction.findOne(_id: ctx.parentID, (err, originalTransaction) -> ...` The exec isn't needed I don't think. See other alerts CR comments.