ENH: parallel context

Currently don't have support for mapping yet, but we can submit system calls or function calls now via IPython. The latter may require state updates on the engines, which is feasible from the current context. It is likely that mapping will go in relatively easily. 

From the shell:

```bash
$ echo -e "profile\tdefault" >> ~/.qiime_config
$ echo -e "n_engines\t2" >> ~/.qiime_config
$ parallel start
```

From within Python:

```python
>>> from qiime.parallel.context import context
>>> context.submit_sync('hostname')
('rl1-1-220-221-dhcp.int.colorado.edu\n', '', 0)
>>> def foo():
...     return "bar"
...

>>> context.submit_sync(foo)
'bar'
>>> def foo(a):
...     return a**2
...
>>> res = context.submit_async(foo, 5)
>>> res.result
25
```

From the shell:

```bash
$ parallel stop
```
Done using pandas (way easier). Check my comment here https://github.com/biocore/scikit-bio/issues/622#issuecomment-52524472 to see why I cannot use the skbio distance matrices.