Add setting for email default priority

This greatly facilitates integration with async backends (e.g.
Celery-based ones).

Fixes #47.
Fixes #51.
Use "DjangoTestSuiteRunner" to in Django 1.6.
Updated README.rst.
Updated EmailBackend to immediately send emails when DEFAULT_PRIORITY is "now".
Email.email_message() should return EmailMessage instance if html_message is empty.
First stab at using a new locking mechanism.
Python 3 compatibility for lockfile.py.
Ignore directory generated by test.
First stab at implementing lazy rendering.
Update README.rst.
Bump version to 0.8.0.
Fixed a bug that causes context to be saved when render_on_delivery is False.
Don't use a dict as default argument.
Simplified the inner workings of mail.send().
Deprecated mail.from_template.
Updated README.rst.
Bump version to 0.8.1.
Fixed test docstring.
Allow user to specify their own context field class.
Updated README.