Vbench and optimize single string parsing
Not sure if this can be trimmed much, but...

```
In [5]: timeit pandas.tseries.tools.dateutil_parse('20000104', datetime(1, 1, 1))
10000 loops, best of 3: 76.6 us per loop
```

Testing should be higher, at the `parse_time_string` level

Time spent mostly in dateutil:

```
In [8]: timeit dateutil.parser.parse('20000104')
10000 loops, best of 3: 66.5 us per loop
```
