new cp setting + vsite_node_insert() = inherit parent theme #3226

#3226 

Ready for review & merge

#### Changes

Parent sites may now opt to have subsite inherit their current theme.

* add new checkbox variable to cp_settings: `vsite_subsite_inherit_theme` default OFF
* add new function `vsite_get_parent()` to vsite module's existing `subsite.inc`
* add function vsite_inherit to modify subsite, called in `vsite_node_insert()`

![screen shot 2013-10-22 at 12 53 16 pm](https://f.cloud.github.com/assets/228733/1383271/850623a6-3b3a-11e3-81c4-7c04feb27024.png)