tests-1.3: add bsn_flow_checksum testsuite

See the Loxigen extension bsn_flow_checksum. This extension maintains checksums
per table and for a configurable number of buckets in each table. The checksums
are sent to the switch in the cookie field of the flow-add. These checksums are
intended to be used for a hash-tree like synchronization scheme.
bsn_flow_checksum.FlowChecksum: check intermediate checksums

Also fixed a bug where 'i' and the checksum were not correspondent.
Merge into master from pull request #112:
tests-1.3: add bsn_flow_checksum testsuite (https://github.com/floodlight/oftest/pull/112)
Merge into master from pull request #114:
update pyloxi to floodlight/loxigen-artifacts@2bdcfd80f3a96740d1b3fb364e... (https://github.com/floodlight/oftest/pull/114)
oft: Add 'disable-ipv6' command line option
packet.py: Skip IPv6 import if IPv6 disabled
Merge into master from pull request #116:
oft: Add 'disable-ipv6' option (https://github.com/floodlight/oftest/pull/116)
update pyloxi to floodlight/loxigen-artifacts@57f3c38ed3f04678bc250d086e150582269280d5