Add pid file argument to the CLI

Immediately, this is useful for running the CLI as part of an
integration test.
In the future it might be helpful when running multiple copies of
gitdocs for different users.
Add aruba based integration testing

In theory it should be possible to do this without cucumber, but
I could not get it to work.

The goal is to get as closed to a full integration test as possible.
This tests adding, syncing, and checking the status of the daemon.

(I have already included the capybara gem, but it is not yet being
used.)