Add download button and download class.
1. add images for download button.
2. add DZDownloadButton class for download button. (almost complete)
3. add DZDownload to wrap DZFileStream. (TODO to complete)
4. set button view class for DZDownloadButton in storyboard.

 Changes to be committed:
	modified:   buttons.ai
	modified:   mypodcast.xcodeproj/project.pbxproj
	new file:   mypodcast/DZDownload.h
	new file:   mypodcast/DZDownload.m
	modified:   mypodcast/Storyboard.storyboard
	modified:   mypodcast/gui/DZDownloadButton.h
	modified:   mypodcast/gui/DZDownloadButton.m
	modified:   mypodcast/gui/UIImage+DZImagePool.h
	modified:   mypodcast/gui/UIImage+DZImagePool.m
	new file:   mypodcast/images/complete-button.png
	new file:   mypodcast/images/complete-button@2x.png
	new file:   mypodcast/images/download-button.png
	new file:   mypodcast/images/download-button@2x.png
	new file:   mypodcast/images/pause-download.png
	new file:   mypodcast/images/pause-download@2x.png