feat #911 Form serialization method

A new method is available in singleton aria.utils.Html. It allows to serialize forms in order to use standard ajax calls to submit them.

Close #911.
fix #864 remove DPI test from css.Units constructor

This commit also adds an additional check for a related error when
the developer is calling `preloadSkinImages` from <HEAD>.

Close #909.
fix #917 Remove unnecessary body focus in aria.templates.TemplateCtxt.$focus

This one broke test.aria.templates.keyboardNavigation.TableNavTestCase on IE <= 8