nx: Minor cleanup

This mostly adds some newlines and makes nxm_entry's __repr__ its __str__.
nx: Add placeholder _nxm_ipv6 class

This class is currently a mangled version of the IPv4 nxm type since
IPAddr6 isn't available in lib.addresses yet.
nx: Fix setting of value in _nxm_ip
nx: Fix setting of nxm masks in __init__
nx: Fix and improve asserts in nxm unpack_new

The data length assertion was just wrong.  Also added a mask length assert.
nx: Fix unpack_new for masked nxm_entrys
nx: Fix typo in _eq() methods
nx: Fix unpacking of register actions

This also refactors some related stuff and adds the _class_for_nxm_header
function.
tests: Add tests for openflow.nicira