Backward support for IPython 2

[gramster]

nosebook wants to upconvert v2 and v3 notebooks to v4 format which doesn't work if v4 is not installed. It would be nice if it still supported v2 installations. That means falling back on conversion failure to v3 format, using nb.worksheets[0].cells instead of nb.cells, and a few other small changes.

[gramster]

Actually there are more changes than I thought, and I'm struggling to understand the internal logic. I can get tests to run in my environment (IPython 2 with a few mods, but I get failures. Here are two sample failures that should be successes. These two cover most of my issues. Any guidance on how to get them to pass would be welcome and I can get a pull request together for the remaining changes.

---

'output_type' value mismatch and 'text' body mismatch:

[
{
"metadata": {},
"output_type": "execute_result",
"text": {
"text/plain": "'The item with key "sampledata/requestlogs/logs_sample.csv" is 3949 bytes'"
}
}
]

[
{
"metadata": {},
"output_type": "pyout",
"text": "'The item with key "sampledata/requestlogs/logs_sample.csv" is 3949 bytes'"
}
]

---

key 'name' mismatch with 'stream':

[
{
"name": "stdout",
"output_type": "stream",
"text": "1402815600.003772,122,200,GET,Interact3\n1402815600.428897,144,200,GET,Interact3\n1402815600.536486,48,200,GET,Interact3\n1402815600.652760,28,405,GET,Interact2\n1402815600.670100,103,200,GET,Interact3\n...\n"
}
]

[
{
"output_type": "stream",
"stream": "stdout",
"text": "1402815600.003772,122,200,GET,Interact3\n1402815600.428897,144,200,GET,Interact3\n1402815600.536486,48,200,GET,Interact3\n1402815600.652760,28,405,GET,Interact2\n1402815600.670100,103,200,GET,Interact3\n...\n"
}
]