Let users select their language

My changes give users the option to set their default language from the "Edit Profile" page. English translations have been reformatted, dutch translations re-written & brand new french translations added.

I will be adding German & Spanish translations some time in the future, unless someone beats me to it.

The profile page has also had a slight makeover:

![RubyGems Profile Design](http://imgkk.com/i/vs2j.png "New Profile Design")

As you can see below, this branch is also up to date with my [commits allowing users to stay signed in for up to a year](https://github.com/rubygems/rubygems.org/pull/678).