Feature/echo rebased

These commits are precisely the same as the last pull request for branch feature/echo, but rebased on top of the current TorProject/ooni-probe/master.

I can confirm that these tests work, and the reports write okay. There is a bug with the test finishing correctly and then print to stdout 100% over and over again, but @aagbsn reports that they have also seen this bug in other development branches, and it is not due to a problem with the echo test. @aagbsn also reports that one of the other pull requests fixes that bug.

Here is the report output from a successful run:
```
###########################################
# OONI Probe Report for echo test
# Sun Feb 24 19:46:37 2013
###########################################
---
options:
  collector: null
  help: 0
  logfile: null
  parallelism: '10'
  pcapfile: null
  reportfile: null
  resume: 0
  subargs: [-f, ./private/lists/ping-me.list]
  test: ./nettests/experimental/bridge_reachability/echo.py
probe_asn: null
probe_cc: null
probe_ip: null
software_name: ooniprobe
software_version: 0.0.10
start_time: 1361735197.0
test_name: echo
test_version: 0.0.4
...
---
input: 8.8.8.8
report:
  8.8.8.8:
    answered:
    - - raw_packet: !!binary |
          RQAAVAAAAAA0AWwJCAgICAoAAJEAAH08AAAAAFo4TVVlRTBQeUE1VWQ2NkRzNzZaTW5qSGhKbGJF
          ZFBBa1F0ekxlTUNnY1FVTnJlM0Q3Q05qYlk5
        summary: IP / ICMP 8.8.8.8 > 10.0.0.145 echo-reply 0 / Raw
    sent:
    - raw_packet: !!binary |
        RQAAVAABAAAZAYcICgAAkQgICAgIAHU8AAAAAFo4TVVlRTBQeUE1VWQ2NkRzNzZaTW5qSGhKbGJF
        ZFBBa1F0ekxlTUNnY1FVTnJlM0Q3Q05qYlk5
      summary: IP / ICMP 10.0.0.145 > 8.8.8.8 echo-request 0 / Raw
    unanswered: []
test_name: test_icmp
test_runtime: 0.04952192306518555
test_started: 1361735197.352775
...
---
input: 192.31.186.46
report:
  192.31.186.46:
    answered:
    - - raw_packet: !!binary |
          RQAAVBE5AAA3Ae2RwB+6LgoAAJEAAEpmAAAAAGFGWHdjazRSRGpxeVN0MElyWFo0U3FHZmJlYVlE
          Y1l1WFNwUVlZc1ZjUzJPSWZ1WE1paXhGcnp3
        summary: IP / ICMP 192.31.186.46 > 10.0.0.145 echo-reply 0 / Raw
    sent:
    - raw_packet: !!binary |
        RQAAVAABAAAZARzKCgAAkcAfui4IAEJmAAAAAGFGWHdjazRSRGpxeVN0MElyWFo0U3FHZmJlYVlE
        Y1l1WFNwUVlZc1ZjUzJPSWZ1WE1paXhGcnp3
      summary: IP / ICMP 10.0.0.145 > 192.31.186.46 echo-request 0 / Raw
    unanswered: []
test_name: test_icmp
test_runtime: 0.21133995056152344
test_started: 1361735197.415798
...
---
input: 72.249.86.184
report:
  72.249.86.184:
    answered:
    - - raw_packet: !!binary |
          RQAAVNc3AAAtAQwwSPlWuAoAAJEAAH7vAAAAAHNVUzZsbnV5ck9IcVRiZXFZeDNTRERLZWttTnEz
          dmR1eDZoUk14UEFBVGFESWMwdUo0SmJ2NFBK
        summary: IP / ICMP 72.249.86.184 > 10.0.0.145 echo-reply 0 / Raw
    sent:
    - raw_packet: !!binary |
        RQAAVAABAAAZAfdmCgAAkUj5VrgIAHbvAAAAAHNVUzZsbnV5ck9IcVRiZXFZeDNTRERLZWttTnEz
        dmR1eDZoUk14UEFBVGFESWMwdUo0SmJ2NFBK
      summary: IP / ICMP 10.0.0.145 > 72.249.86.184 echo-request 0 / Raw
    unanswered: []
test_name: test_icmp
test_runtime: 0.21760010719299316
test_started: 1361735197.633252
...
---
input: 38.229.72.16
report:
  38.229.72.16:
    answered:
    - - raw_packet: !!binary |
          RQAAVNMiAAA4ATYBJuVIEAoAAJEAAEm5AAAAAG96Nnh6bEFiMkpqS090bk5PMkMyWmNDaTk2WFF3
          Qm02enh6cFUyQTNiMXpSd1FXRll4MHBHVkxS
        summary: IP / ICMP 38.229.72.16 > 10.0.0.145 echo-reply 0 / Raw
    sent:
    - raw_packet: !!binary |
        RQAAVAABAAAZASgjCgAAkSblSBAIAEG5AAAAAG96Nnh6bEFiMkpqS090bk5PMkMyWmNDaTk2WFF3
        Qm02enh6cFUyQTNiMXpSd1FXRll4MHBHVkxS
      summary: IP / ICMP 10.0.0.145 > 38.229.72.16 echo-request 0 / Raw
    unanswered: []
test_name: test_icmp
test_runtime: 0.21865415573120117
test_started: 1361735197.861163
...
---
input: 127.0.0.1
report:
  127.0.0.1:
    answered: []
    sent:
    - raw_packet: !!binary |
        RQAAVAABAAAZAaOmfwAAAX8AAAEIANIvAAAAAGk3Vm9MWlNWYlExcTF2Qm1RMHBXU3U0aFVCYUdU
        QUd0RXRFcEUwSUFpbHVZZkZBWWdod1NKT1pn
      summary: IP / ICMP 127.0.0.1 > 127.0.0.1 echo-request 0 / Raw
    unanswered: []
test_name: test_icmp
test_runtime: 10.041443109512329
test_started: 1361735198.08863
...
---
input: 192.168.0.1
report:
  192.168.0.1:
    answered: []
    sent:
    - raw_packet: !!binary |
        RQAAVAABAAAZAdZuCgAAkcCoAAEIAPEzAAAAAHNFZzN5Y3dsc2pDdEY5QXpYcU5MdEh4TWRYdEdR
        ZWMxQ3pOYWRPMVRxYmNKNjdvbjBrVnNrZEhS
      summary: IP / ICMP 10.0.0.145 > 192.168.0.1 echo-request 0 / Raw
    unanswered: []
test_name: test_icmp
test_runtime: 10.042280912399292
test_name: test_icmp
test_runtime: 10.042280912399292
test_started: 1361735208.137619
...
```