Genericising the Invoice model

Tests now pass for member signup processing
Changes to invoicer_support for refactoring
Make all Invoicers new style
Updating the atendee monitor and features to handle the new Invoicer interface
Improving signup processing feature
Updating Invoicer and tests to use the new style
Set VCR to record once
New style Invoicer complete!
Remove the old-style documentation for Invoicer and replaced with proper new docs
Updated "I should not be added to the invoicing queue step" to check against the invoice description

We're not passing order numbers from Eventbrite in any more, so need to
check against the invoice description
Merge pull request #154 from theodi/feature-106-refactor-invoicer

Refactored Invoicer to be more generic and work with membership stuff