Fix client/engine APIs and configuration syntax
for selectable engine/database to execute queries

Squashed commit of the following:

commit 6407aae129fbe706a876c33df0bceaa6b79bc96a
Author: TAGOMORI Satoshi <tagomoris@gmail.com>
Date:   Sun Jan 26 15:31:09 2014 +0900

    remove debug prints

commit 230e6d4aed7ad09a9a927f9b3ea1084d34cad57e
Author: TAGOMORI Satoshi <tagomoris@gmail.com>
Date:   Sun Jan 26 15:24:13 2014 +0900

    fix to have query execution options in each engines

commit 6f80b7806ab5a3df693d4c66aabb4f30a3fbc0ef
Author: TAGOMORI Satoshi <tagomoris@gmail.com>
Date:   Sun Jan 26 14:59:23 2014 +0900

    fix to hide when init

commit 177df86c63f06d9a216664cd1301ba4751cf0cc2
Author: TAGOMORI Satoshi <tagomoris@gmail.com>
Date:   Sun Jan 26 14:58:33 2014 +0900

    fix to hide engine/db when missing

commit c526d01a9dbeceb9eef3084a9a05c519286edb7c
Author: TAGOMORI Satoshi <tagomoris@gmail.com>
Date:   Sun Jan 26 14:55:49 2014 +0900

    add debug prints

commit 6fff8b7a372957bee990f861868a6a5a4998e905
Author: TAGOMORI Satoshi <tagomoris@gmail.com>
Date:   Sun Jan 26 14:38:53 2014 +0900

    fix missing engine label

commit 91c775f0895ec59e3e15552cbe39474ff93d295f
Author: TAGOMORI Satoshi <tagomoris@gmail.com>
Date:   Fri Jan 24 19:09:12 2014 +0900

    fix syntax

commit 6229dfd9f2acfe23f3b371f8074ed8a6d932b1ef
Author: TAGOMORI Satoshi <tagomoris@gmail.com>
Date:   Fri Jan 24 19:07:10 2014 +0900

    specify internal server error explicitly

commit 4123dd8fcbe9344db83eb25ab2dd35bae044f0aa
Author: TAGOMORI Satoshi <tagomoris@gmail.com>
Date:   Fri Jan 24 19:04:47 2014 +0900

    re-fix

commit 5f9fba6c3b72368205186ab1271d0789b3701eec
Author: TAGOMORI Satoshi <tagomoris@gmail.com>
Date:   Fri Jan 24 19:00:33 2014 +0900

    revert previous

commit 0d1ba9eb7586eec36ee2c44606a9fc511215c83a
Author: TAGOMORI Satoshi <tagomoris@gmail.com>
Date:   Fri Jan 24 18:59:16 2014 +0900

    new style express application

commit b250cbfa76073b6f8e48f68ddb01e6b6e13ca61c
Author: TAGOMORI Satoshi <tagomoris@gmail.com>
Date:   Fri Jan 24 18:57:24 2014 +0900

    fix error logging

commit 2368450be5a145f0390e214d3e743a218b153020
Author: TAGOMORI Satoshi <tagomoris@gmail.com>
Date:   Fri Jan 24 18:54:24 2014 +0900

    add exception dumper

commit af0d9e8519c15d941ed1ee225cdfc179cd02c395
Author: TAGOMORI Satoshi <tagomoris@gmail.com>
Date:   Fri Jan 24 18:35:34 2014 +0900

    fix visibility

commit c9b53a742f53d3ecf1a9cd1d36e8389cce632e33
Author: TAGOMORI Satoshi <tagomoris@gmail.com>
Date:   Fri Jan 24 18:34:05 2014 +0900

    fix char size

commit 9335d33287574a1be291d0cdcc19c8c23fda3afd
Author: TAGOMORI Satoshi <tagomoris@gmail.com>
Date:   Fri Jan 24 18:27:53 2014 +0900

    fix to show engine selector at start

commit 59008238ae2d35fb0a8f1264a0ddbc17aa8cb774
Author: TAGOMORI Satoshi <tagomoris@gmail.com>
Date:   Fri Jan 24 18:18:16 2014 +0900

    fix

commit f1e6fa6464cc935b2d39ff8937599edc48936336
Author: TAGOMORI Satoshi <tagomoris@gmail.com>
Date:   Fri Jan 24 18:13:50 2014 +0900

    fix typo

commit c0e78670740bbe2fe7d5ff6cb8f4251c73a6c744
Author: TAGOMORI Satoshi <tagomoris@gmail.com>
Date:   Fri Jan 24 18:11:42 2014 +0900

    fix variable context and its name

commit a36a711a8d19a7feb17d064569457a19495258c3
Author: TAGOMORI Satoshi <tagomoris@gmail.com>
Date:   Fri Jan 24 17:13:14 2014 +0900

    refine internal/external APIs with engine-database pairs