Add flat prior option to binom_conf_interval.  Fix bugs with scalars.

* Add flat prior option to binom_conf_interval and binned_binom_proportion
* Fix bug where jeffreys prior crashed when given scalar values of k.
* Added unit test for flat prior and for scalar jeffreys prior.
* Update docs and put in 0.4 changelog
Fix typo (fucntion)
Comments from @kbarbary

* Fix docs
* Try to return input shape more faithfully.  Add some unit tests
  of that behavior.
Change the way shapes are handled in binom_conf_interval

* Rather than explicitly reshape the output, treat scalars
  as a special case and let the ufuncs deal with broadcasting.
  Doesn't seem to change any aspect of the output.
Merge pull request #2157 from aconley/binom_flat

Add flat prior option to binom_conf_interval.  Fix bugs with scalars.