Added buttons to the home screen, committed resources to git.

briar-android/res/drawable-xhdpi/content_email.png

briar-android/res/drawable-xhdpi/device_access_accounts.png

briar-android/res/drawable-xhdpi/green_bulb.png

briar-android/res/drawable-xhdpi/grey_bulb.png

briar-android/res/drawable-xhdpi/navigation_cancel.png

briar-android/res/drawable-xhdpi/navigation_refresh.png

briar-android/res/drawable-xhdpi/social_chat.png

briar-android/res/drawable-xhdpi/social_person.png

briar-android/res/drawable-xhdpi/social_share.png

briar-android/res/values/strings.xml

...-android/src/net/sf/briar/android/HomeScreenActivity.java