Add overview to release notes and other edits

[MRG+1] Fix for KeyError in robots.txt middleware

Release notes: edits on "Py3 Support" and "New features" sections

[MRG+1] Remove __str__ and __repr__ from settings, introduce copy_to_dict()

[MRG+1] PY3 fix downloader slots GC

[MRG+1] Fix bug on XMLItemExporter with non-string fields in items

Suggested by @digenis
see http://ipython.readthedocs.org/en/stable/api/generated/IPython.lib.pretty.html?#extending

Settings instances as dict's are easier to print or pretty print in the shell

Fixes #1732

[MRG+1] shell command's ability to open local files + tests

[MRG+1] Include tests for non-string items to Exporters