add a serial pool, refs #1

 * Adds a `SerialPool` that allows you to run your map in one process for debugging purposes.
 * Adds a coverage target.
 * Makes the performance test pass on my box (dev14) by bumping it down :(

Up to you folks whether you'd want to consolidate `DebugPool` and `SerialPool`.

```
Name                        Stmts   Miss Branch BrPart  Cover   Missing
-----------------------------------------------------------------------
vimap/__init__                  0      0      0      0   100%
vimap/exception_handling       24      4      2      0    85%   37-41
vimap/pool                    123     14     46      5    89%   85-92, 110, 150, 167, 235-238
vimap/queue_manager            56      3     18      4    91%   89, 99, 102
vimap/real_worker_routine      68     16     10      2    77%   30, 41, 47-53, 74, 78, 93-97, 105-108
vimap/testing                  52      1     14      0    98%   51
vimap/util                     14      0      6      0   100%
vimap/worker_process           13      0      2      0   100%
-----------------------------------------------------------------------
TOTAL                         350     38     98     11    89%
```