Add :checkout option to allow checkinable pod when using --no-clean

The svn downloader uses export instead of checkout by default since
https://github.com/CocoaPods/CocoaPods/pull/473.

But having a "checkin-able" pod is still usefull for the Pods developpers even
if it has been aggreed to be a minority-use case.
(from https://github.com/CocoaPods/CocoaPods/issues/245 discussion).
Merge branch 'svn-checkout' of https://github.com/yalp/cocoapods-downloader into yalp-svn-checkout

* 'svn-checkout' of https://github.com/yalp/cocoapods-downloader:
  Add :checkout option to allow checkinable pod when using --no-clean

Conflicts:
	lib/cocoapods-downloader/subversion.rb
[SVN] Small refactor
Merge branch 'yalp-svn-checkout'

* yalp-svn-checkout:
  [SVN] Small refactor
  Add :checkout option to allow checkinable pod when using --no-clean