Add Letvcloud support, as in #422

There's one problem with the extractor:

The main extractor is failing to send ```yuntv.ltev.com``` to the right extractor, ```letvcloud```.

I am feeling that fixing this problem is over my ability, so I 've finished everything else, hoping someone else would like to fix it up.

Now you can call ```letvcloud_download_by_vu``` to download Letvcloud's video.

P.S: I have to set the file permission to 755 in order to edit them. And I am sure you would need to re-edit them later, since the bug with the main extractor is left untouched. I would not say it is not working, but the effect is lower than my expectation.