Add H904: don't wrap lines using a backslash

Hacking originally had a soft preference for () versus \ based line
continuation, but this has lead to unnecessary review churn. So in order
to reduce churn be stricter and don't allow \ based line continuation.

Related ML thread
http://lists.openstack.org/pipermail/openstack-dev/2013-November/019024.html

Change-Id: I010623344bc2c4bf86051f86a96afb3611c90342
Merge "Add H904: don't wrap lines using a backslash"