[Merge] lp:~penk/gallery-app/sdcard into lp:gallery-app

[Penk Chen]

Loop through /media sub directories and append it to mediaDirectories list if exists. 

This is a related merge proposal of camera-app to add SD card support:
https://code.launchpad.net/~penk/camera-app/sdcard/+merge/231568
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https://code.launchpad.net/~penk/gallery-app/sdcard/+merge/231587
Your team Ubuntu Phablet Team is requested to review the proposed merge of lp:~penk/gallery-app/sdcard into lp:gallery-app.