Added: Correct regexp for WWDC2014 (fixes #11)

The format of the video urls is different this year.

```
http://devstreaming.apple.com/videos/wwdc/2014/712xx1pl2u942g2/712/712_hd_writing_energy_efficient_code_part_2.mov?dl=1
```

- uses underscores instead of hyphens
- might contain text after `123_hd`, see `712_hd_writing_energy_efficient_code_part_2`
- format is lower-case (so we need to use `grep -i`)

Note: I closed my previous pull-request #13 because it did an unnecessary if/else - this pr only modifies the regular expression.