Arcyon: add 'task-create' command for Maniphest

Add a bare-bones command for adding tasks to Maniphest.

Test Plan:
Add new command to smoketest
$ ./precommit.sh
aoncmd_arcyon: exit w/ return value of subcommand
aoncmd_taskcreate: enforce non-empty title

Although the conduit API currently permits creation of a task with an
empty title, this appears to be a mistake.  Enforce a non-empty title on
the command-line.

Use print function to be forwards-compatible with Python 3.

Test Plan:
$ ./precommit.sh
try to create tasks with empty titles:
$ arcyon task-create ''
$ arcyon task-create ' '
arcyon task-create: support 'description'

Add support for writing the 'description' field when creating a new task
with 'arcyon task-create'.

Test Plan:
$ ./precommit.sh
tried create tasks with descriptions
Arcyon create-task: support 'priority'

When creating a task with 'arcyon task-create', support specifying
priority by name, e.g. 'wish' or 'unbreak_now'.

Test Plan:
extend
$ ./precommit.sh
tried creating with different priorities