Add event lister, and refactor cukes

This was originally two commits, but I gitted wrong and now it's one. Sorry. 0:30 with @pikesley
Resolve merge conflict
Work out Xero API, first cut of scenarios

2:00 with @Floppy
Oops, shouldn't have done that
Merge branch 'refs/heads/master' into feature-21-invoicing
Added doc block for EventLister
Removed Xero keys
Set up resque error reporting into redis
formatting fix
Separate development gems into a group

0:05
Use standard naming for step directory
Add guard for cucumber

0:10
Add spark to speed up guard

0:10
ignore private keys
Extend invoker interface to accept hashes of data for user, event and payment
Refactor features into separate files to keep them tidy

0:10
Split eventbrite and zero steps

0:05
Auto-filter sensitive keys

0:05
Add a place for post-test cleanup to go

Tests should be idempotent when run without VCR. 0:05
Added a whole load of cucumber scenarios for invoicing

3:00