Fix bug where rewriting the hash with a new hash_size would make it a no-op
Ignore build and test remnants
Print internal error messages to stderr

Makes logs a bit easier to scan through, as errors are in a separate
stream.
Merge pull request #20 from spotify/krka/hash_size

Fix bug where rewriting the hash with a new hash_size would make it a no...
Merge pull request #23 from stephenmathieson/print-errors-to-stderr

Print internal error messages to stderr
Merge pull request #22 from stephenmathieson/ignore-test-remnants

Ignore build and test remnants
Fix bug where _write_full() fails if interrupted.

If write() is interrupted, this would raise SPARKEY_INTERNAL_ERROR instead
of resuming/retrying the write.