Fixed compile breakage

Warning, this is my first ever pull request so it might be stupid.

When attempting to build cberl with erlang R16B I get:

==> cberl (compile)
Compiled src/cberl_nif.erl
compile: warnings being treated as errors
include/cberl.hrl:7: opaque type handle() is not exported

I fixed this by changing the line to -type handle() :: Binary()  from -opqaue handle() in cberl.hrl.

This allows me to build.  I haven't been able to test the result, because when I launch erl, it seems poolboy isn't starting:

$ erl -pa ebin
Erlang R16B (erts-5.10.1) [source-05f1189] [64-bit] [smp:8:8] [async-threads:10] [hipe] [kernel-poll:false]

Eshell V5.10.1  (abort with ^G)
1> cberl:start_link(cberl_default, 5).
** exception error: undefined function poolboy:start/2