Preparing for 3.5

3.5 adds TwentyTwelve, so let's take it out. Additionally, IIRC, TwentyTen should not be delivered on fresh installs, but I'm not sure. In either case, since 3.4 is still latest stable, leaving that option in.

Also, added check to ensure the directory exists before attempting to remove it, so trunk installs work with the twentyten options selected and 3.4 installs work with twentytwelve selected.

* Adds option to remove TwentyTwelve. 
* Adds check to confirm directory is present before deletion for twentyten/eleven/twelve