doc: add battery

Fixes #114

- reorganize common classes in fake API such as Display, Speaker and Battery.
- This does not change the user API

source/ev3brick.rst

@@ -50,3 +50,10 @@ Display
 .. automethod:: ev3brick.display.text
 
 .. automethod:: ev3brick.display.image
+
+Battery
+-------
+
+.. automethod:: ev3brick.battery.voltage
+
+.. automethod:: ev3brick.battery.current

source/signaltypes.rst

@@ -111,3 +111,19 @@ frequency: Hz
 Sound frequencies are expressed in Hertz (Hz).
 
 For example, you can choose the frequency of a :meth:`beep <.ev3brick.sound.beep>` to change the pitch.
+
+.. _voltage:
+
+voltage: mV
+--------------
+Voltages are expressed in millivolt (mV).
+
+For example, you can check the voltage of the :meth:`battery <.ev3brick.battery.voltage>`.
+
+.. _current:
+
+current: mA
+--------------
+Electrical currents are expressed in milliampere (mA).
+
+For example, you can check the current supplied by the :meth:`battery <.ev3brick.battery.current>`.