DRP 0.4.0

Django REST Pandas 0.4.0 brings a few new features as well as confirmed compatibility with Django 1.9.

• #15 - Ability to override JSON format via URL keyword (via @kevinmickey)
• #17 - Unstacked, Scatter, and Boxplot serializers for common charting uses, generalized from wq.db.contrib.chart.
• #14 - Better documentation and tests for "simple" use case