Store and use the feed's language.

Feed's language can be specified via RSS <language> element or xml:lang
attribute set on the root element in Atom feeds. It is then used to set
lang attribute on the article container in the feed view.
Detect right-to-left languages and set text direction accordingly.

Solves issue #34.
Specify language of feed view document based on selected locale.

This sets the overall language of the document and applies to such
elements as day headers in headlines view and the empty view
messages. Feed articles have their language set separately,
as declared by feeds themselves.

This is necessary to correctly handle upper-case text transform
in Greek (issue #41) and possibly other language quirks too.