Implement episode filter for rss downloader

**Still needs testing**

Partially implements: #223

Syntax looks like this: `SxEP;EP-EP;EP-;`

Season is mandatory non-zero value.
Episode is mandatory non-zero value.
Episodes can be chained with semicolons: e.g. `1x1;2;8-12;24-;` will match 1, 2, 8 through 12 and 24 through infinity (actually 09999) episodes of season one.
Filter string must end with a semicolon.

![epf](https://f.cloud.github.com/assets/1910559/848203/ff3d1a42-f456-11e2-8af5-3c2b8af2cf44.png)

~~P.S. Still needs tooltip for text box.~~ **Done.**