Merge pull request #132 from tsareg/pass_facing_mode_through_create_local_tracks_options

Add ability to pass facingMode as option to JitsiMeetJS.createLocalTracks()

doc/API.md

         4. micDeviceId - the deviceID for the audio device that is going to be used
         5. minFps - the minimum frame rate for the video stream (passed to GUM)
         6. maxFps - the maximum frame rate for the video stream (passed to GUM)
+        7. facingMode - facing mode for a camera (possible values - 'user', 'environment')
 
 * ```JitsiMeetJS.enumerateDevices(callback)``` - __DEPRECATED__. Use ```JitsiMeetJS.mediaDevices.enumerateDevices(callback)``` instead.
 * ```JitsiMeetJS.isDeviceListAvailable()``` - __DEPRECATED__. Use ```JitsiMeetJS.mediaDevices.isDeviceListAvailable()``` instead.

modules/RTC/RTCUtils.js

  * @param {string} options.desktopStream
  * @param {string} options.cameraDeviceId
  * @param {string} options.micDeviceId
+ * @param {'user'|'environment'} options.facingMode
  * @param {bool} firefox_fake_device
  */
 function getConstraints(um, options) {
         } else {
             // Prefer the front i.e. user-facing camera (to the back i.e.
             // environment-facing camera, for example).
-            constraints.video.facingMode = 'user';
+            // TODO: Maybe use "exact" syntax if options.facingMode is defined,
+            // but this probably needs to be decided when updating other
+            // constraints, as we currently don't use "exact" syntax anywhere.
+            constraints.video.facingMode = options.facingMode || 'user';
         }
 
         constraints.video.optional.push({ googLeakyBucket: true });