Add nested buckets.

This commit adds the ability to create buckets inside of other buckets.
It also replaces the buckets page with a root bucket.

Fixes #56.
Merge branch 'master' into nested-keys

Conflicts:
	db_test.go
	tx_test.go
Upgrade import/export to use nested buckets.
Merge pull request #127 from benbjohnson/nested-keys

Add nested buckets.