Performance improvements to WebAgg backend
Address @pelson's comments in the PR.
Merge pull request #2261 from mdboom/webagg-improvements

WebAgg performance improvements
Fix bug reading favicon as a utf-8 encoded file.
Make logo_sidebar_horiz.png less wide
Mention six in the what's new and INSTALL docs
Merge pull request #2267 from mdboom/mention-six-in-install

Mention `six` in what's new and install docs
Merge pull request #2265 from mdboom/webagg-python3-encoding-issues

WebAgg favicon serving error in Python 3
If pygobject is installed, but not the gobject introspection bindings for gtk, it throws a RuntimeError, not an ImportError, and we need to catch that.
Merge pull request #1939 from mdboom/gtk-test-without-x11

GTK error, failed building on command line
Fix PyCXX check -- if import passes, we should assume that including it will also
Override stdout differently on Python 2 and 3
Merge pull request #2283 from mdboom/stdout_override

Incorrect overriding of sys.stdout
Merge pull request #2282 from mdboom/pycxx-fix-check

Extend search path for PyCXX headers