Defaulting to a DATABASE_URL with user defined

so that netgo/cgo-disabled builds don't get runtime errors with the
default settings.
Merge remote-tracking branch 'origin/master' into defaulting-to-database-url-with-user

Conflicts:
	Makefile
Merge pull request #47 from modcloth/defaulting-to-database-url-with-user

Defaulting to a DATABASE_URL with user defined