Structure and validate PulsarExec arguments

Depends on #12.

### Allow to pass arguments to task creation
One can pass additional arguments when starting a pulsar/capistrano task with:
```
pulsar <app> <env> <task> -s foo=123 -s bar=456
```

We should allow to pass these to the API endpoint when creating a new task.
How about adding an additional URL parameter `args` which contains a JSON-hash?
```
POST /:app/:env?task=:task&args={"foo":12}
```