* Implemented LinearSmoothedValueAtomic class based on the
  JUCE 4.0 library' LinearSmoothedValue class but with some improvements
  that were suggested by Timur Doumler during his JUCE Summit 2015
  talk.
* LinearSmoothedValueAtomic is very convenient to use with float values
  that need linearly smoothed behaviour (like volume, etc.) and which
  are shared between several threads (i.e. audio and GUI).
* This class guarantees we won't have any race conditions and it will
  remains lock-free because of using std::atomic inside it.

* Implemented ButtonGroupManager.
  It is responsible for positioning of buttons, any animations
  during switching, etc.
* Added MaterialButtonLookAndFeel class which implements
  flat button look and feel following Google Material Design Guidelines.

Fixed: Changing data directory in GUI did not take effect if a recording had been started (and stopped) before. Only the first directory before the first recording was used. Fixed: EventBroadcaster was linking to Debug version of open-ephys.lib in Release mode

Fixed: Changing data directory in GUI did not take effect if a recording had been started (and stopped) before. Only the first directory before the first recording was used. Fixed: EventBroadcaster was linking to Debug version of open-ephys.lib in Release mode

Reverted some extraneous changes to VS2013 build files made in commit a2ba811d