indexing, allowing tolerances to account for imprecise nature of FP
numbers.
Initial implementation, rather complicated.
A simple rounding-based implementation can be put in later.
Includes initial test.
* First module: file_db to store filename, fingerprint (md5sum) and basic
stats (mtime, size). Additional fields can be specified as well.
* Will use sqlite as the backend for now.
* API is rather generic so the underlying database engine can be replaced.