add numba example to enhancingperf.rst
![enhancing performance pandas 0 16 1-81-g676cb95 documentation 1](https://cloud.githubusercontent.com/assets/953992/7959452/33d0364a-09c7-11e5-867c-b71bb27d085d.png)

+1 !

With the latest numba, I think you can do the empty array allocation inside the jitted function, instead of passing it as an argument. I find this a bit easier to follow, but of course in this way it also works with older versions.
 @jorisvandenbossche for some reason, sphinx wouldn't render the decorators in the ipython blocks (e.g. the `@jit`. very odd. (it would work with verbatim, but then you have to repeat the entire code in a suppress block to actually have it executed).
 @jorisvandenbossche I put the allocation back. I think its ok. In the next version of numba, this will just be 'faster' (e.g. its all done in nopython mode). But seems to work ok.
 Note that the version of Numba that will be able to do the memory allocation in nopython mode will be coming out today.
 @seibert hmm, this is seeming to execute in nopython mode already though? (I guess except for the allocation, which prob doesn't matter in perf as its a single event), right?
 It does not execute in nopython mode with current stable version of numba, but because of the loop lifting of the main part of the function, it does not make that much of a difference I think.

@seibert yes, I was referring to that version, but forgot that it was only a beta release announcement that I saw
 Right, loop-lifting means that allocating memory in nopython mode is not as exciting as you might initially think.  :)
 @jreback Could you replace the cython timings with a `:verbatim:`, too? Otherwise the numbers will get out of sync.

Generally this looks good to me
 @shoyer updated to use all verbatims..
