Added an activity for creating groups, moved some classes.

briar-android/AndroidManifest.xml

briar-android/res/values/strings.xml

.../src/net/sf/briar/android/groups/CreateGroupActivity.java

...id/src/net/sf/briar/android/groups/GroupListActivity.java

...et/sf/briar/android/groups/WriteGroupMessageActivity.java

...id/src/net/sf/briar/android/identity/LocalAuthorItem.java

.../sf/briar/android/identity/LocalAuthorItemComparator.java

.../sf/briar/android/identity/LocalAuthorSpinnerAdapter.java

...c/net/sf/briar/android/invitation/AddContactActivity.java

...src/net/sf/briar/android/invitation/NetworkSetupView.java