added mode in Preference class
modified openConfigureCamera in KLTVideoDisplayActivity class, when the mode is not CAMERA_MODE, this method will simply return null
added new button for opening video in main activity
added video mode support in main activity
Merge branch 'add-support-for-video' of https://github.com/pan-long/KLTTrackerOnAndroid into add-support-for-video

Conflicts:
	app/src/main/java/sg/edu/nus/comp/klttracker/KLTMainActivity.java
	app/src/main/java/sg/edu/nus/comp/klttracker/KLTVideoDisplayActivity.java
	app/src/main/java/sg/edu/nus/comp/klttracker/Preference.java
Merge branch 'add-support-for-video' of https://github.com/pan-long/KLTTrackerOnAndroid into add-support-for-video