ability to use default Mandrill subject and from fields
prepare for PR to mainstream (tests, docs, ...)
fix test
Merge branch 'feature/use-mandrill-template-defaults' of git://github.com/nikolay-saskovets/Djrill into nikolay-saskovets-feature/use-mandrill-template-defaults
use_template_subject and use_template_from

Change "clear_subject" and "clear_from" to clarify expected usage.
Update docs.
Acknowledge contribution.
Merge branch 'nikolay-saskovets-feature/use-mandrill-template-defaults'
Travis: lose deprecated pip --use-mirrors
Travis: update Django 1.7 to b4