Detect feed languages

This fixes issues #34 and #41, and probably other possible issues with various languages that require special care.

Note that this patch detects language based on RSS ```<language>``` element and Atom ```xml:lang``` attribute on the root element. Feeds can also specify language separately for each item by setting ```xml:lang``` on individual items, but it's rather rare and supporting it would require considerably more work, as the feed parser doesn't provide that.

@Tanriol, would you like to review?