Redid inbox as list of SmsConversation objects

Created SmsConversation class to hold info about a conversation.
Created custom adapter to adapt a ArrayList<SmsConversation> into a
ListView.
Rewrote refreshConversation() to use these new functions.
Renamed Sms class to SmsMessage (to help differentiate from
SmsConversation).
Set up view recycling for conversation list

Implemented view recycling for listview population in the conversation
list.