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

briar-android/.gitignore

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

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

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

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

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

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

briar-android/res/drawable-hdpi/notification_icon.png

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

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

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

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

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

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

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

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

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

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

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

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