title: scipy stats binom cdf returns nan
body: <p>If I understand correctly, the <code>cdf</code> for a <code>scipy.stats</code> discrete distribution should return the sum of the probabilities of the values up to the parameter given.</p>
  
  <p>Thus, <code>scipy.stats.binom(7000000000, 0.5).cdf(6999999999)</code> should return something almost exactly 1, because in 7 billion trials, with a 50/50 chance, the probability of getting a success on 7 billion minus 1 of them or less is pretty much certain.  Instead, I get <code>np.nan</code>.  In fact, for any value provided to <code>.cdf</code> EXCEPT 7 billion itself (or more), I get back <code>np.nan</code>.</p>
  
  <p>What is going on here?  Is there some limit to the numbers that <code>scipy.stats</code> distributions can handle that is not in the docs?</p>