Adds another error test case for reading nonexistent files
Improves storagedriver tests for Move command semantics

Ensures that Move will properly overwrite the file at the destination
location.
Also checks that Move of a nonexistent source file will NOT delete the
file at the destination.
Fixes storagedriver Stat test

Checks Stat on the directory before creating the file to make sure that
it does not exist
Properly cleans up after the test.
Improves storagedriver concurrency testing

Creates trees instead of flat files for TestConcurrentFileStreams

Adds TestConcurrentStreamReads, which writes a large file (smaller in
Short mode), and then ensures that several concurrent readers properly
read their portions of the file with random offsets
Attempts to fix go vet for circle
Merge pull request #837 from BrianBland/ng-storagedriver-tests

Various storagedriver test additions/improvements