Convert shipping_total and tax_total to strings

As of Django 1.6 Decimal's cannot be stored in sessions.  Convert shipping_total and tax_total to strings before storing in the session.
Adding reportlab 2.7 as a dependency, with a comment that this is due to a bug in pisa, and also that this may not work on Python 3.x
Merge
Fix tax test
Update pdf invoices to use xhtml2pdf. Closes #180.
Added Danny Sag to AUTHORS.
Travis fix for installing pypdf
Travis fix for installing pypdf
Added variation sku as search fields to products.
Potential fix for encoding currencies on Windows with python 3.
Added www.jnkbows.com to sites in README.
README formatting
Clean up some imports