Tweaked variable names in `pyvows.runner`
Comments added to `pyvows.runner`
Made `pool` a class-level attribute
Organized code in `VowsParallelRunner.run_context_async()`

My hope is that this is step 1 towards a better refactoring in the future.  For now, it’s good simply to break down pieces of long functions into smaller ones.
In `VowsParallelRunner`: `_file_info_for()` → `_get_file_info_for()`

Renamed for name-consistency with other methods
Re-alphabetized pseudo-private methods in `VowsParallelRunner`
Added comments explaining call order of `run()`
Refactored `elapsed()` method into `pyvows.utils`
Alphabetized imports in `pyvows.runner`
Extracted methods from `VowsParallelRunner` to module functions (for now…)

There’s almost certainly a better place for these functions—perhaps `pyvows.utils`.

I’m leaving them here for now, since I’ve been moving a lot of code lately, and I’m starting to get nervous. :)
Removed superfluous variable in `VowsParallelRunner.run()`
Streamlined a few `except` blocks

Given:

```
    exc_type, exc_value, exc_traceback = sys.exc_info()
```

And:

```
    except Exception as e:
        ...
```

The following are true:

```
bool(e == exc_value)
bool(e is exc_value)
```

Nifty, huh? ☺
Merge pull request #83 from Zearin/refactor/runner

Semi-refactoring of `pyvows.runner`