Fix Payday::PdfRenderer.number_to_currency using implementation from Monetize. from_bigdecimal
Add --color option to rspec
Add Travis CI configuration file
Support custom renderers and custom font/size for PdfRenderer

- Move rendering specs out of invoice_spec
- Update README.md
Add support for a globally configurable renderer
Use :font_style for bold table cells instead of :inline_format
Make overriding specific PdfRenderer styles easier

- Separate table data from presentation
- Introduce template method pattern structure so derived classes can override specific parts of the rendering process
- Remove unnecessary method Invoiceable#each_detail
Don't render a subtotal line unless it differs from the total
Implement more general invoice-specific translations mechanism

Now all translations can be made invoice-specific e.g. 'Hours' or 'Days' instead of 'Quantity' in the table heading depending on billing period.

Note: backward compatibility with shipping_description and tax_description has not been maintained
Issues #15/#16: Don't require shipping_rate or tax_rate
Allow I18n string interpolation from within PdfRenderer
Allow null line item quantities

We can then decide how to display them using LineItemable#display_quantity