Added description of preferences.
Updated todo.txt.
Repaired link.
Reordered strings.  No real change.
Changed prefs related to locations.
Removed outcommented check on mock location because I can't seem to
get it to work.
Consistentified member field names.
Changed 'previous' to 'logged' in various identifiers.
Removed m_logged_location and m_logged_cell_info.  Simply always
retrieve them from the prefs.
Moved json encoding / decoding of GsmCellInfo to GsmCellInfo.
Moved location json encoding/decoding to separate methods.
Changed logged location methods to static so getLoggedLocation() can be
called from the outside.  Use that in MainActivity.
Added prefs_current_location, which is for now a duplicate of prefs_logged_location.
Update the displayed location unless it's really worse.