make background page use latest available IE version

The background window in IE comes up in IE7 mode; this is apparently the
default for embedded internet explorer controls. This change makes the
page load up using the latest available IE, though perhaps not for pages
that would be considered 'intranet' pages.

The following stack overflow queries seem relevant, found via searching
for related keywords on using IWebBrowser2:
http://stackoverflow.com/questions/6914664/iwebbrowser2-object-uses-ie7-version-instead-of-the-ie-version-installed-on-the
http://stackoverflow.com/questions/4097593/how-to-put-the-webbrowser-control-into-ie9-into-standards
Remove dialog-question.ico from project
Merge pull request #27 from makern/dll-injection

Fix DLL injection bitness mismatch
Merge pull request #28 from makern/ie7-fixes

Fix button and popup not working on IE7
Merge pull request #29 from makern/ie9-fixes

Move extension button to Tools Bar in IE9+
Merge pull request #30 from Sessionbox/ie-background-use-latest

make background page use latest available IE version
Merge pull request #32 from makern/head-script

Insert scripts after meta tags
Merge pull request #33 from makern/feedback-dialog

Remove Apture feedback dialog (IE)
Consolidate customizable build time settings into vendor.h