Fix typo in %reset docstring. Closes gh-256.

Fix bug with execution of naked multiline strings. The actual bug fix was a trivial one-line change, made here. The rest of the commits in this series improve our testing machinery and clean up related code. The actual fix was just calling the run_source instead of the run_code method, which should only be called with compiled code objects.

Limit special-casing of _ variable to doctests. In doctests, _ *must* be special-cased and removed from the user's namespace because otherwise Python won't set it. But we were doing this special-casing unconditionally, making it impossible to write unittests that checked the _ variable after cell execution. This commit makes the special-casing of _ apply only in the doctest execution (where it's needed) and leaves normal unittests alone.

Remove code that is not needed once python 2.6 is our minimal version.

Improve docs and comments of some internal tools, and of testing code

Small cleanup of dead debug code

DOC: Fix typo in a recommended command in gitwash.

logging implemented kernel's messages

Fix bug where bare strings would be silently ignored in input. Bare strings are parsed by the AST module as the docstring of the 'module' (in our case, the string of user input), so they need to be special-cased.

BUG: Use arg_split instead of shlex.split