MCS Library Enhancement for #21 (#25)

## New features

 * As requested in #21, Provide new method `setServerValue(value)` for all controller data channels. Developers may use this method to update the MCS/MCSLite server-side values. Once the value is successfully set, the channel becomes both `valid()` and `updated()`.

## Changes

 * `MCSGPSController` now have a new method `value()` which returns an `MCSGPSValue` object that is a struct of latitude, longitude and altitude. The exisiting methods such as `latitude()` are left unchanged.
 * `MCSPWMController::value()` has changed:
    * It now returns a `MCSPWMValue` object instead of the **duty cycle** field.
    * A new method `dutyCycle()` is added to retrieve the field seperately.
 * All controller channels are now inherited from a `MCSControllerBase<T>` template class that provides a common implementat for `value()`, `setServerValue()` and `_update()` logics. This reduces some redundant code implementations. Note that **display channels** are left unchanged.