FARADAY_HTTP_BACKEND now affects RssAgent and PostAgent too.
Set a global default User-Agent value via an environment variable.

Names starting with HTTP_ should be avoided in the context of Web app,
hence the rather verbose name `DEFAULT_HTTP_USER_AGENT`.
Use content_tag.
Add a hover menu to the "Agents" nav link.
Merge pull request #440 from knu/agents_hover_menu

Add a hover menu to the "Agents" nav link.
Tweak gem dependencies to improve platform support for *BSD and Solaris.

* ffi: pull in unreleased fixes from GitHub master on *BSD.

* tzinfo: require >=1.2.0 to gain support for *BSD and Solaris,
  eliminating the need for the tzinfo-data gem.
Merge branch 'platform_support'