Add the ability to queue indexes.

For a current project we deal with a lot of products. Therefor the indexing process takes a long time but does finish properly, great!

The problem arose when the internetconnection wasn't as stable as it should have been, meaning that the SSH-pipe broke and the indexing stop. Of course we can /dev/null the script, but that still requires me to do it via the terminal. This frustrated me a long time that it is possible to reindex the indexes in the admin panel, but you aren't able to schedule them.

I've added the ability to schedule an index so it is executed when the cron runs. You can now select `Queue Reindex` for each index or massQueue them all at once. The queue ability is depended on Aoe_Scheduler (normal schedule module doesn't have the scheduleNow method). If Aoe_Scheduler isn't installed this functionality is hidden.

If you guys can accept the pull request or at least provide feedback, that'd be cool :)