Add ability to select features with exp distrib

Rather than having a sharp cutoff, use a smooth cutoff.
Ideally, we could should att this for the other two selection
methods also, but they're complicated and I'm lazy.
export the exp_distrib option to user-space.
fix typos in the last changeset ...

aiee .. I already pushed
Merge pull request #179 from linas/moses

Moses exponential feature distribution