Add 'checksum' command to CLI

[benbjohnson]

This command computes the LTX checksum of a single database file. This can be used for debugging a checksum mismatch between the database file and the last LTX file.

Usage

Running against a single database:

$ ltx checksum my.db
fe13f1d0a5d4d2f4

Providing no arguments shows the help:

$ ltx checksum
The checksum command computes the LTX checksum for a database file.

Usage:

	ltx checksum PATH

Providing more than one arguments returns an error:

$ ltx checksum my.db other.db 
too many arguments