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
* PEP8ify

@kbarbary should look at this.