Added a preference menu that allows MongoDB credentials to be inputted instead of being hardcoded.  The preferences menu is accessible from the main activity, depending on the device via the top right settings icon or the bottom left settings button.  Tested on Nexus 5 v4.4. and Galaxy S3 v4.3.

Conflicts:
	AndroidManifest.xml
	src/com/nightscout/android/dexcom/DexcomG4Activity.java
	src/com/nightscout/android/upload/UploadHelper.java
ported pref menu from rnpenguin/dexcom-uploader