Move m_storageId from FsStoragePlugin up to ancestor StoragePlugin
FSStoragePlugin: reduce scope of txCancelled variable
Fix array index out of bounds in StorageTracker::copy()

Result row leftover from previous query had only one item, causing
resultSet[i][1] to fail.
Fix FSStoragePlugin::copyObject() for cross-storage file copying
Check free space in FSStoragePlugin::copyObject()
StoragePlugin::copyObject(): NULL destinationStorage means the same storage

Also refreshed method documentation a bit.
Merge pull request #24 from xhaakon/interstorage-copy-object

Fix FSStoragePlugin::copyObject() for cross-storage file copying