Early version of message threading

Missing:
- UI support for threading when polling
- code to upgrade existing installations
- UI elements to switch from/to threaded display mode
- threading of messages with same subject
Nasty hack to retain threads in MessageListFragment when polling
Worked around NPEs when the activity has been detached
Don't display thread count in thread view
Created search package to hold the framework and moved over the SearchSpecification class ( refactor ).
Adding the 3 core classes for the search framework. ConditionsTreeNode, LocalSearch and SearchSpecification.
Add new search logic to the MessagingController and LocalStore classes.
Made thread_root a searchable field of the message table. This can be used to display threads.
Fixed wrong construction of leaf sets and unrightfully ignored exceptions.
Added two convenience methods to avoid dealing with exceptions all the time.
Avoid needless nullpointers.
Add new SearchAccount class and move it together with SearchModifier to the search package ( refactor ).
Change visibility of SearchModifier class and members.
Add static methods to create unifiedInbox and allMessages accounts.
Changed all the calls to MessageList to work using the new LocalSearch class. These are all tested and working changes.
First attempt at making the fragmented messagelist and remote search play nice with the new search framework. Works partially.
Switched to ContentProvider and CursorLoader to display the message list
Merge branch 'new_search_framework' into content_provider

Conflicts:
	src/com/fsck/k9/fragment/MessageListFragment.java
Whitespace cleanup
Revert changing the default value for debug logging