Arcyd: add 'add-phabricator' command

Arcyd is hard to setup.

Make it a bit easier to get started by providing an 'add-phabricator'
command which create an appropriate configuration file for the
specified parameters.

Validate the parameters by attempting to connect to the phabricator
instance.

Test Plan:

Use 'arcyd add-phabricator' where appropriate in
./testbed/arcyd/exercise_arcyd.sh

$ ./precommit.sh
Arcyd: add 'add-repo' command

Arcyd is hard to setup.

Make it a bit easier to get started by providing an 'add-repo'
command which creates an appropriate configuration file for the
specified parameters.

Clone the repository and test writing to some of the refs we expect to
manage with Arcyd.

Test Plan:

Use 'arcyd add-repo' where appropriate in
./testbed/arcyd/exercise_arcyd.sh

$ ./precommit.sh
Arcyd: add 'start' command

Arcyd is hard to get running.

Provide a 'start' command which will launch an instance of Arcyd if
the current directory has been set up by the new 'init' and 'add*'
commands.

Test Plan:

Use 'arcyd start' where appropriate in
./testbed/arcyd/exercise_arcyd.sh

$ ./precommit.sh
Arcyd: add 'init' command

Arcyd is hard to stop gracefully.

Provide 'arcyd stop' to make the options discoverable and documented.

Test Plan:
Use 'arcyd stop' where appropriate in ./testbed/arcyd/exercise_arcyd.sh
$ ./precommit.sh
testbed/arcyd/test_shell.sh: use new cli

Convert test_shell.sh to use the new Arcyd CLI

Test Plan:
$ ./testbed/arcyd/test_shell.sh
WIP: Arcyd: use pidfile to improve start/stop