Add some testing of the backup functionality.

[a-p-]

This adds some testing of the backup functionality.

It tests using a single Step(-1) call, as well as multiple Step() calls.

It also does some testing of the PageCount() and Remaining() methods.

[coveralls]

Coverage increased (+3.4%) to 69.817% when pulling 3a5d0a7 on a-p-:backup-tests into c3e9588 on mattn:master.

[coveralls]

Coverage increased (+3.4%) to 69.817% when pulling 3a5d0a7 on a-p-:backup-tests into c3e9588 on mattn:master.

[mattn]

Cool.