Merge branch '2.x'

# Conflicts:
#	composer.json

Merge pull request #13 from tomirons/develop

Merge recent changes into master

Added tests and support for PHP 5.6

Update package to use new email markdown

Refactor the `items` method, deprecate the `item` method.

Incorporate subtotal, tax, and shipping into invoices

Fix template issues and add plain text templates.

Initial release