fix UnicodeEncodeError for dbref

Fix "UnicodeEncodeError: 'ascii' codec can't encode character ..." error in case dbref contains non-ascii characters
fix UnicodeEncodeError for dbref

Fix "UnicodeEncodeError: 'ascii' codec can't encode character ..." error in case dbref contains non-ascii characters
test for #278 issue
Updated field iteration for py2.5
Merge remote-tracking branch 'origin/pr/278'
Update AUTHORS and changelog (#278)
Updated queryset
Merge branch 'master' into 0.8

Conflicts:
	AUTHORS
	mongoengine/base.py
	tests/test_dereference.py