Limitations

Introduced some resource constraints to close out #36. Note that these may be incompatible with the host kernel. Docker by default will steamroll right on through, warning you along the way:

```
[W 140928 09:49:41 orchestrate:178] [u'Your kernel does not support swap limit capabilities. Limitation discarded.']
```

This introduces the following options:
```
  --cpu_shares                     Limit CPU shares, per container
  --mem_limit                      Limit on Memory, per container (default 512m)
```