Add :checkout option to allow checkinable svn pod when combined with --no-clean

The svn downloader uses export instead of checkout since
https://github.com/CocoaPods/CocoaPods/pull/473.

But having a "checkin-able" pod is still usefull for some Pods developpers even
if it has been aggreed to be a minority-use case.
(from https://github.com/CocoaPods/CocoaPods/issues/245 discussion).

Currently with Git, it is just by using `--no-clean` to `pod install` or `pod update`.
With this options, SVN can also have this feature.