first implementation of decompress_ptr

My first shot at the ``decompress_ptr`` that we discussed on the mailinglist this morning.

The benchmark has been upgraded:

https://gist.github.com/esc/5326039

On my my two core laptop (Intel(R) Core(TM)2 Duo CPU L7500 @ 1.60GHz) I get:

2.53304290771
0.657001018524

Which has changed from:

2.53254199028
1.03268194199

On a 24 core machine (4 x Six-Core AMD Opteron(tm) Processor 8431) I get:

1.52762794495
0.315052986145

which was previously:

1.55133485794
0.590444087982

Essentially using ``decompress_ptr`` makes the compress decompress roundtrip roughly 50% quicker, at least from a first impression.

Again, I will provide  a convenience function in ``toplevel.py``. One thing I haven't figured out yet, is what to return? Perhaps the number of bytes written?