add cmd /c in open_url for Windows

This was necessary for opening a chart in the browser to work for me (64 bit Windows 7, Julia 0.3.0-prerelease) otherwise I was getting ```ERROR: could not spawn `start 'C:\Users\Tony\AppData\Local\Temp\julia2.html'`: no such file or directory (ENOENT)```.

Separate issue, but the version of this package that's currently tagged in Metadata does not work due to changes in DataFrames. I had to do `Pkg.checkout("GoogleCharts")` but that extra step isn't always obvious to new users.