Switch default IPython directory back to ~/.ipython

As discussed at the dev meeting, we care more about a consistent default
across platforms than half-heartedly following the XDG specs.
Update tests for IPython.utils.path
Update doc references to .config/ipython
update man page to reflect XDG disrespect :)
remove XDG link
add note to "what's new"
a couple of more stray XDG / .config references
Merge pull request #8 from ivanov/ipython-dir-default

main change is adding a what's new entry
Merge pull request #4457 from takluyver/ipython-dir-default

Use ~/.ipython as default config directory