Add Import

We don't need to worry about caching.
That happens on read.
Clean up miscellaneous tests and error returns
More singularity of purpose in import_test
Code cleanliness updates

- Factor out cache busting code
- Make explicit WithLock, WithRLock in function signatures
Merge branch 'master' into import

Conflicts:
	diskv.go
Merge pull request #18 from peterbourgon/import

Add Import method