Support PSRadioGroupSpecifier

Fixes #129.

Radio groups work like regular groups and therefore get their own sections. However, one specifier represents multiple rows, so the specifier is used multiple times for each radio group value.

Because the selection and reloading code is identical to `PSMultiValueSpecifier`, I've extracted it from `IASKSpecifierValuesViewController` into a separate class (`IASKMultipleValueSelection`). There is one selection instance per `IASKSpecifierValuesViewController`, and one selection *per section* in `IASKAppSettingsViewController`.

To make things easier, I've changed the data source to consistently use `IASKSpecifier`. Previously, a raw `NSDictionary` was used for groups, and `IASKSpecifier` for everything else.