Method cache

So I recently wanted to use the wrapper pool, for external interface preservation reasons. 
Saw your warning about method_missing, so I added some code to dynamically define the proxy methods to the connection pool objects on the fly, a la ActiveRecord's old finder methods.
Now you pay the price only once per unique method calls, after that the method is 'cached'.

That said, after I finished & tested the change, I benched it. On my 1.9.3 install, it really dosent make much difference, because the `method_missing` code wasn't much slower than the `with` to begin with(100000 calls):
<pre>
wrapper:                12.440000   0.060000  12.500000 ( 12.511956)
cached_wrapper:    12.120000   0.050000  12.170000 ( 12.180331)
with:                      11.880000   0.050000  11.930000 ( 11.977355)
</pre>

I suspect its pretty quick anyway because the dispatch chain to method_missing from BasicObject is really short. 

So, I don't know, its up to you whether you think this patch is worth it or not.