First commit after moved to bitbucket.
1. add new class DZTableViewCell, DZTableViewController to wrap code related with swipeable utility button and action sheet.
2. move removeDownload from DZItem extension to DZDownloadList.
3. remove reference to DZItem in DZDownloadButton, think about another way to reference DZItem from download button.
[TODO]
no.3 above.
Refine DZTableViewCell and DZTableViewController.
1. remove updateUtilityButtons from DZTableViewCell, no need to update explicitly now.
2. remove call to updateUtilityButtons from DZFeedItemCell.
3. refine cell actions for now-playing item.
4. make swipeRightCell property in DZTableViewController readonly.
 Changes to be committed:
	modified:   mypodcast/gui/DZFeedItemCell.m
	modified:   mypodcast/gui/DZTableViewCell.h
	modified:   mypodcast/gui/DZTableViewCell.m
	modified:   mypodcast/gui/DZTableViewController.h
Move download related code to DZItem category.
1. create DZItemDownload category for DZItem.
2. move download related code in DZItemDownload.
3. move ULR to DZFileStream mapping to DZItemDownload.
4. remove DZDownloadList.
5. remove URL to DZFileStream mapping from DZFileStream.
Move download related code to DZItem category.
1. create DZItemDownload category for DZItem.
2. move download related code in DZItemDownload.
3. move ULR to DZFileStream mapping to DZItemDownload.
4. remove DZDownloadList.
5. remove URL to DZFileStream mapping from DZFileStream.
Refine item-cell mapping and download button.
1. add DZItem category DZItemCellMapping to manage mapping from feed item (DZItem) to a collection (NSSet) of table view cell (DZFeedItemCell).
2. move DZDownloadButton action target from DZFeedViewController to DZFeedItemCell.
3. invoke download logic in DZFeedItemCell with its feedItem property.
Merge branch 'master' of bitbucket.org:richarddzh/mypodcast

Conflicts:
	mypodcast.xcodeproj/project.pbxproj
	mypodcast/gui/DZFeedItemCell.m
Hide utility button when other cell is tapped.
Add filter for downloaded or half downloaded item.
1. add download segment to segmented control.
2. add downloadProgress and downloadStatus property to DZItemDownload category.
3. add filter logic in DZFeedViewController.
Add UIRefreshControl to DZFeedViewController.
- add refresh control by set table view controller's refresh enabled.
Add search utility to DZFeedViewController.
Fix some bugs in table view search.
1. clear search text string when table reloaded by filtering.
2. scroll to selected search result (rather than selecting, see TODO).
[TODO]
Why selection of table cells will override the background color of its subviews.
When selecting a table cell, the contained buttons change background color to white.
Add download information to DZFeedItemCell label.