Add support for merged videos to formats

I've added the new optional field "sub_formats" to formats. It's an array which contains the format used for video and the format used for audio.
The function ```_sort_formats``` in ```extractor/common.py``` assigns merged formats a preference of -1000 if ffmpeg/avconv is not available.
I've also added the function ```_add_merged_formats``` to ```extractor/common.py``` which automatically generates merged formats and makes it very easy for IEs to support merged formats. (E.g. in YoutubeIE: 68c925b)
The source files of merged videos get deleted after merging. (3ab9e41)
I've moved the function ```select_format``` from ```YoutubeDL.py``` to ```utils.py``` that's the biggest change in terms of lines (+40-40).
```
$ youtube-dl-git -F "http://www.youtube.com/watch?v=QAUzWtLMnU0"
...
[info] Available formats for QAUzWtLMnU0:
format code extension resolution  note 
171         webm      audio only  DASH audio , audio@ 48k (worst)
140         m4a       audio only  DASH audio , audio@128k
160         mp4       144p        DASH video , video only
242         webm      240p        DASH video , video only
133         mp4       240p        DASH video , video only
243         webm      360p        DASH video , video only
134         mp4       360p        DASH video , video only
244         webm      480p        DASH video , video only
135         mp4       480p        DASH video , video only
247         webm      720p        DASH video , video only
136         mp4       720p        DASH video , video only
248         webm      1080p       DASH video , video only
137         mp4       1080p       DASH video , video only
160+140     mp4       144p        audio@128k
17          3gp       176x144     
242+171     webm      240p        audio@ 48k
133+140     mp4       240p        audio@128k
36          3gp       320x240     
5           flv       400x240     
243+171     webm      360p        audio@ 48k
134+140     mp4       360p        audio@128k
43          webm      640x360     
18          mp4       640x360     
244+171     webm      480p        audio@ 48k
135+140     mp4       480p        audio@128k
247+171     webm      720p        audio@ 48k
136+140     mp4       720p        audio@128k
22          mp4       1280x720    
248+171     webm      1080p       audio@ 48k
137+140     mp4       1080p       audio@128k (best)
```