Added a simple test to verify it works

So, I got further along now, but I get a segmentation fault if I run this test; I assume this should work?
```
==> gen_driver (compile)
==> gen_driver (eunit)
======================== EUnit ========================
module 'gen_driver_test'
module 'gen_driver'
module 'basic_test'
  basic_test: setup_and_teardown_test...Segmentation fault (core dumped)
tojans@ubuntu:~/develop/erlang/fdb-erlang/deps/gen_driver$ 
```

This it the test code:
```erlang
-module (basic_test).

-include_lib("eunit/include/eunit.hrl").

setup_and_teardown_test()->
  {ok, Drv} = gen_driver_test:start_link(),
  gen_driver_test:stop(Drv).

basic_test() ->
  {ok, Drv} = gen_driver_test:start_link(),
  timer:sleep(500),
  {ok, Result} = gen_driver_test:sum(Drv,[1,2,97]), 
  ?assertEqual(100.0, Result),
  gen_driver_test:stop(Drv).
```

AFAIK the segmentation fault is in `gen_driver_test:stop`.