Add meta page checksums.

This commit adds checksums to the meta pages on every write. When the
database loads, it verifies the checksums on the meta pages and returns
an error if either one is corrupt.

In the future, it should fallback to the previous meta page but for right
now it just hard fails. This is at least preferable to opening the database
and getting a random error or further corruption.

Fixes #25.
Merge pull request #113 from benbjohnson/meta-checksum

Add meta page checksums.