if there are no interfaces to ping, there are no packets sent
if a test fails or exits with an error, run cleanup as a precaution
add cli usage information
support wireshark versions greater than 1.11
Add more information for test condition failure
Use server receive rate rather than client send()/buffering rate

Fixes #412
Merge pull request #409 from cdburkard/patches/cleanup_tests

if a test fails or exits with an error, run cleanup as a precaution
Merge pull request #400 from cdburkard/patches/fixEmptyPing

Mininet crashes when running ping between two hosts with no interfaces
Merge pull request #410 from cdburkard/patches/tshark_walkthrough_1404

support wireshark versions greater than 1.11 in test_walkthrough
Merge pull request #411 from cdburkard/devel/cli_usage

add cli usage information
Try to prime the pump to avoid PACKET_INs during iperf test

Background: the reference controller is reactive and installs exact
match rules. By attempting to start a telnet session we make sure that
the ARP caches and TCP flow rules are set up (in one direction at
least) before the test starts.

This is intended to help with #413