Added TimberTest::testGetPostPreview

Test creates a post and a preview and sets the appropriate GET params for previews. It eventually checks if it gets the content from the autosave, instead of from the original post.
Add Timber::maybe_set_preview and call it in Timber::get_posts

Added the method maybe_set_preview, largely based on WPs internal _set_preview function. Checks if the GET params call for a preview and if the current user is allowed to edit the current post. It scans the posts array for the previewed post and if present replaces it by the last autosave by the current user (again, if available).
Merge branch 'post-previews' of github.com:mgmartel/timber into mgmartel-post-previews

* 'post-previews' of github.com:mgmartel/timber:
  Add Timber::maybe_set_preview and call it in Timber::get_posts
  Added TimberTest::testGetPostPreview