Prevents caches that were never used being created as side effect of BlobCache.Shutdown()

[jcomtois]

Fixes issue #192.

Adding a null "ShutdownBlobCache" that can be used to prevent caches that were never used from being created upon BlobCache.Shutdown(). I set a static bool when Shutdown is called, then add a check of that bool to the null coalescing chain before a new instance would be created.

Please be kind, this is my first pull request on any project EVER.

[anaisbetts]

Fix the indents here and on the other property declarations, so it looks like:

int Foo { 
  get { return 5; } 
}

[anaisbetts]

Hey @jcomtois, this is a very good first PR! Make an extra commit or two to your branch to fix up these little things and I'll hit the merge. You don't need to close / resubmit on GitHub, just add extra commits to your branch

[jcomtois]

Thanks, @paulcbetts. Let me know if there are any more tweaks required. I'm a heavy user of ReactiveUI and Splat and just starting to use Akavache so I'm pleased that I can make a contribution back when your stuff has definitely made my life easier.

[anaisbetts]

Thanks @jcomtois!