Loads of fixes

* Cross-provider serialization errors (e.g last commit you could not duplicate SQL items with Rhino installed!)
* Filtering fixes
* Cross-provider copying and moving errors as appropriate (e.g. moving a SQL item into a serialized folder and vice versa). Unfortunately this is only possible at present to copy an item _from_ SQL _to_ Rhino, because the SQL provider can't find Rhino source items. Moving is nigh impossible it seems.
Simplified and optimized loading code

Instead of a tree walker, simply iterates over recursive file paths
Also added parallel enumeration - this dropped the load time on 1499 serialized items (on SSD) from ~250-300ms to ~120-170ms. YMMV if it's a single spindle HDD though.
Update readme
FileSystemWatcher support for cache invalidation

NOTE: at the moment triggering the watcher causes TOTAL sitecore cache annihilation. This is a bit broad, but it works for the moment and there's a TODO comment to go fix it later.

Also updated readme