Allow inheritance of all MongoEngine documents.
Merge pull request #48 from ParlantTechnology/tastypie-0.9.15-compatibility

TastyPie 0.9.15 compatibility
Ups. Bugfix.
Ups again.
For newer versions of MongoEngine `_reset_already_indexed` is not needed anymore.
Missing default attribute.
Better fix for blank fields.
Fix tests for polymorphic resources with related resource name.
Set Mongoengine user for Django 1.5 in tests.
Added CONTRIBUTING file. Fixes #41
Should use lambda for default.
Show that we have to_many relations in schema.
Tests for datetime field.
Version bump.
Removed TODO.
Indentation in travis.yml.
MongoEngine 0.8.2 broke backwards compatibility so let's stick with 0.8.1 for now.
Version bump.
Test more versions.
Correct Travis CI environment variables.