Merge other pull requests, measure boomerang time, fix rt.start bugs

[bluesmoon]

There are multiple commits in this pull request. These include the following:

• Merge in @edgarklein's change in https://github.com/yahoo/boomerang/pull/37
• Few modifications to @edgarklein's pull
• Merge in @okuryu's change in https://github.com/yahoo/boomerang/pull/36
• Merge in @xzyfer's change in https://github.com/yahoo/boomerang/pull/35
• Merge in @dguzzo's change in https://github.com/yahoo/boomerang/pull/32
• Fix bug where rt.start was set to navigation even if using google toolbar or older chrome versions
• Measure time it takes to load up boomerang itself
• Set undefined and null values to '' in the beacon instead of sending 'undefined'
• Support prerender for IE as well
• Support W3C spec visibility change event
• Do not use cookie time if user got to page using back button (and we know it)

[okuryu]

+1