battery plugin: Add StateFS backend.

battery plugin: Add StateFS backend.

Pull-Request: #1795

Squashed commit of the following:

commit 8f4841f381a59e1f1afd3101f93d84079866e8fa
Author: rinigus <rinigus.git@gmail.com>
Date:   Wed Sep 7 11:10:27 2016 +0300

    battery plugin, statefs backend: use power to report values

commit bab1a17ca6d6b2a21c64eb7c5a2b0cdb658f50bc
Author: Rinigus <rinigus.git@gmail.com>
Date:   Tue Sep 6 23:53:51 2016 +0300

    battery plugin, statefs backend: adjusting type names and small corrections

commit 6428720c330f7773f3e335c8426edb9d2f8f3139
Author: Rinigus <rinigus.git@gmail.com>
Date:   Sat Sep 3 12:15:19 2016 +0300

    battery statefs backend: cleanup

commit 6039a12774d3933a88bcc4612c92699d3a816f97
Author: Rinigus <rinigus.git@gmail.com>
Date:   Sat Sep 3 07:45:45 2016 +0300

    battery plugin: clang-format for statefs backend

commit 55ec4952dbc5643af335ca9d9802df438bf4a006
Author: Rinigus <rinigus.git@gmail.com>
Date:   Sat Sep 3 07:37:52 2016 +0300

    battery plugin: adjusting read_statefs call

commit 4a41e8f1b88f58ef09b0c8702e752d9d8f793e38
Author: Rinigus <rinigus.git@gmail.com>
Date:   Fri Sep 2 23:43:49 2016 +0300

    battery: statefs backend saves as battery plugin

commit bbc25d26663cb727b884e3931e6a909a71dac160
Author: Rinigus <rinigus.git@gmail.com>
Date:   Fri Sep 2 23:19:56 2016 +0300

    battery_statefs: define read function for statefs backend

commit 352cf7d9c2146e0960b8b688dfafcbdd54a789f1
Author: Rinigus <rinigus.git@gmail.com>
Date:   Thu Sep 1 23:36:35 2016 +0300

    battery_statefs: incorporating into battery plugin

commit 65e9d416c028d919efc2d901b782bb465c82f56b
Merge: c9eca12 bb98f71
Author: Rinigus <rinigus.git@gmail.com>
Date:   Thu Sep 1 23:08:36 2016 +0300

    battery_statefs: merge updates from upstream

commit c9eca127455571e8adf1f5c5f4290e56b793040b
Merge: a6b4c85 6cf7955
Author: Rinigus <rinigus.git@gmail.com>
Date:   Sun Jul 24 14:29:57 2016 +0300

    fix configure.ac

commit a6b4c851ae3e257faad417737065aacba1db9303
Author: Rinigus <rinigus.git@gmail.com>
Date:   Mon Jul 18 23:51:44 2016 +0300

    return statements

commit d31678bb5b3d9707063986ace17e5d7ba03976b5
Author: Rinigus <rinigus.git@gmail.com>
Date:   Mon Jul 18 23:46:19 2016 +0300

    changes suggested by @rubenk

commit 56785979aa49319a0a4622ff7ece0cc1ad1fd9a6
Author: Rinigus <rinigus.git@gmail.com>
Date:   Mon Jul 18 23:40:51 2016 +0300

    changes suggested by @rubenk

commit 6cbf0d03a9404f560d346ea770e06e38ed11206f
Author: Rinigus <rinigus.git@gmail.com>
Date:   Fri Jul 15 10:03:58 2016 +0300

    statefs_battery config