Add DB.NoSync option for bulk loading.

This commit adds the DB.NoSync flag to skip fsync() calls on each commit. This should only
be used for bulk loading as it can corrupt your database in the event of a system failure.

Initial tests show it can provide a 2x speed up for sequential inserts.
Merge pull request #225 from benbjohnson/no-sync

Add DB.NoSync option for bulk loading.