use_member/1 helper function

Hey Seth,

I found that most of the time the usage of the pool was pretty simple and hence I thought I'd throw this into the main pool so that it can be reused. No pressure to include it, just thought you might like to take a look in case you find it useful and think it should be included in your main repo.

Here are the details of the commit:

> In many cases the process pool will follow the standard pattern of
> Get->Use->Return, and hence there'll be a lot of code scattered
> through applications which follows this pattern. Rather than leave
> the responsibility of following this pattern to the caller, the
> use_member/1 function was added to cater for this primary case.
> 
> This function catches exceptions and returns the member to the pool
> with an indication of error so that the process can be restarted.
> If the function succeeds then the process is returned untouched.
> 
> Added some documentation to the README so that it's a bit more
> visible to future users.


Cheers mate
OJ