Commit before pull (don't remember the changes made)
Merge branch 'dev/solo' of https://github.com/lca4/unison-android into dev/solo
SoloTracksFragment: displays tracks of playlist
FIX Group History menu appears only when needed

Group History is invisible by default (defined in menu.xml).
Visible in:
* GroupsActivity
* GroupsMainActivity
Sets GroupsMainActivity launch mode as singleTop
FIX home button misbehaviour

* If still on HomeActivity, doesn't close the app
* If in GroupsMainActivity, leaves the group
HomeActivity extends SherlockActivity

Instead of AbstractFragmentActivity, which was not suitable since the Home is
not intended to host fragments.