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 tests in MiniTest

Includes testing through the CLI for share management, daemon status and
share synchronization.

I also made a small change to the existing tests and how ENV['TEST'] is
set to ensure that the integration tests run with a persisted database,
but the previous tests still run in memory.
Merge pull request #71 from acant/feature/cucumber

Integration testing from the CLI