Added function evhtp_unbind_socket

evhtp_unbind_socket stops listen socket, and deallocs all memomry
allocated by evhtp_new.

Valgrind with code:
int
main(int argc, char ** argv) {
    evbase_t * evbase = event_base_new();
    evhtp_t  * htp    = evhtp_new(evbase, NULL);
    evhtp_callback_t * cb;

    cb = evhtp_set_cb(htp, "/test", testcb, NULL);
    evhtp_callback_free(cb);
    evhtp_bind_socket(htp, "0.0.0.0", 8080, 1024);

    return 0;
}

==11359== Memcheck, a memory error detector
==11359== Copyright (C) 2002-2011, and GNU GPL'd, by Julian Seward et al.
==11359== Using Valgrind-3.7.0 and LibVEX; rerun with -h for copyright info
==11359== Command: ./test_basic
==11359==
==11359==
==11359== HEAP SUMMARY:
==11359==     in use at exit: 12,136 bytes in 57 blocks
==11359==   total heap usage: 60 allocs, 3 frees, 12,230 bytes allocated
==11359==
==11359== LEAK SUMMARY:
==11359==    definitely lost: 632 bytes in 1 blocks
==11359==    indirectly lost: 9,248 bytes in 9 blocks
==11359==      possibly lost: 0 bytes in 0 blocks
==11359==    still reachable: 2,256 bytes in 47 blocks
==11359==         suppressed: 0 bytes in 0 blocks
==11359== Rerun with --leak-check=full to see details of leaked memory
==11359==
==11359== For counts of detected and suppressed errors, rerun with: -v
==11359== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 4 from 4)

Valgrind with code:
int
main(int argc, char ** argv) {
    evbase_t * evbase = event_base_new();
    evhtp_t  * htp    = evhtp_new(evbase, NULL);
    evhtp_callback_t * cb;

    cb = evhtp_set_cb(htp, "/test", testcb, NULL);
    evhtp_callback_free(cb);
    evhtp_bind_socket(htp, "0.0.0.0", 8080, 1024);

    evhtp_unbind_socket(htp, 1);
    free(htp);
    event_base_free(evbase);
    return 0;
}

==11264== Memcheck, a memory error detector
==11264== Copyright (C) 2002-2011, and GNU GPL'd, by Julian Seward et al.
==11264== Using Valgrind-3.7.0 and LibVEX; rerun with -h for copyright info
==11264== Command: ./test_basic
==11264==
==11264==
==11264== HEAP SUMMARY:
==11264==     in use at exit: 8,216 bytes in 2 blocks
==11264==   total heap usage: 60 allocs, 58 frees, 12,230 bytes allocated
==11264==
==11264== LEAK SUMMARY:
==11264==    definitely lost: 24 bytes in 1 blocks
==11264==    indirectly lost: 8,192 bytes in 1 blocks
==11264==      possibly lost: 0 bytes in 0 blocks
==11264==    still reachable: 0 bytes in 0 blocks
==11264==         suppressed: 0 bytes in 0 blocks
==11264== Rerun with --leak-check=full to see details of leaked memory
==11264==
==11264== For counts of detected and suppressed errors, rerun with: -v
==11264== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 4 from 4)
Merge branch 'release/0.4.15' into develop
Merge pull request #38 from jbro/master

Added function evhtp_unbind_socket
Use POSIX 200112 for *NIX based builds.
Merge branch 'develop' of github.com:ellzey/libevhtp into develop
Drain before letting ownership change.
jfkdal
Document evhtp_set_timeouts().
Merge branch 'develop' of github.com:ellzey/libevhtp into develop
Release v0.4.16
Merge branch 'release/0.4.16'
Merge branch 'release/0.4.16' into develop
Added cmake option for disabling regex.
Cleanup compiler warnings.
Fixed EVHTP_DISABLE_SSL compile.
Release v0.4.17, see ChangeLog for details.
Merge branch 'release/0.4.17'
Merge branch 'release/0.4.17' into develop
Added some function documentation.
Check for null deref on request check.