Get the Post's date so the Invoice can use it.

Needs more monkey patching, but necessary to allow Invoice to find
effective tax rate.
Pull 'effective' logic out into a Utils module for use elsewhere.
Get effective tax rate from _data file, replacing hard-coding.

In implementing this we need to depend on Jekyll 1.3 or higher as
this is when the _data directory support was added.
Set invoice's default tax type from _config.yml if present.
Add guard rule to monitor changes to test/fixtures.

Need to ensure we exclude _site though, otherwise guard will loop
continuously when the test causes the _site directory to be
regenerated.
Factor out the site regeneration in test_converter.rb
Run all tests when the changes ones have passed.
Tidy up attr declarations.
Make tax rate a percentage throughout.