Cold boot handling

[Zeegaan]

Fixes #16

Notes

• Moves Rebuild indexes notification handler into core, as that is a core responsibility
• Adds a UmbracoApplicationStartedNotification handler to examine, that will handle cold boot
• On boot we now check for every index
  • Does the index exist
  • Is there anything in the index
• So if the index doesn't already exist or there is nothing in it, we determine that as a cold boot, and we will try to rebuild.

How to test

• With your own test site referencing the source code
• Create some content, stop the site, set a breakpoint in the new RebuildIndexNotificationHandler
• Start the site and assert that it skips rebuilding
• Stop the site again, now clear out the index files
• Start the site and assert that now it rebuilds, and you have index files again.