MRG set min_df in fe.text back to 1

`min_df=2` is simply too confusing for users, since it causes trivial examples with fewer than three documents (and often with a few more) to fail. Many scikit-learn questions on StackOverflow pertain to this, and the fact that I've been recommending `TfidfVectorizer` for general-purpose document comparison (rather than learning) makes this worse.

As @ogrisel remarked somewhere, expert users will know how and when to set this parameter.

Not sure if there was already an issue about this on the tracker?