Drop 2.6 and 3.2 from Travis tests
Remove extra assert methods needed for Python 2.6
Update docs for Python version support
Drop nested() function for context managers
Remove _total_seconds helper function
Remove Python 2.6 workaround
remove workaround for 2.6 support
irunner use python -m IPython to launch subprocess with matching interpreter

Closes gh-4013
Merge pull request #4002 from takluyver/drop-26-32

Drop Python 2.6 and 3.2
don't 'restore_bytes' in from_JSON

it makes no sense to turn base64-encoded unicode strings into base64-encoded byte strings.  I can't think why we do this, but we should be very careful about testing before merging this fix.

fixes the issue tested in #4036.
update test_extract_output
use cast_unicode in nbformat
test that PNG / JPEG output data are unicode
Test no incorrect 'data:image/png;base64,b' in html files.
stonebig's test shouldn't fail anymore
note that restore_bytes is never used