add mode to stats:poisson_conf_limits

[hamogu]

'kraft-burrows-nousek'** This is a Bayesian appraoch which allows
for the presence of a known background :math:B in the source signal
:math:N.

http://adsabs.harvard.edu/abs/1991ApJ...374..344K

[pllim]

Appveyor failed in Python 3.4

        elif interval == 'kraft-burrows-nousek': 
            if conflevel is None: 
                raise ValueError('Set conflevel for method {0}. (sigma is ignored.)'.format(interval)) 
            conflevel = np.asanyarray(conflevel) 
            if np.any(conflevel <=0) or np.any(conflevel >= 1): 
                raise ValueError('Conflevel must be a number between 0 and 1.') 
>           if background < 0: 
E           TypeError: unorderable types: list() < int() 

astropy\stats\funcs.py:698: TypeError 
= 1 failed, 7464 passed, 96 skipped, 69 xfailed, 3 xpassed i

[mirca]

expected*

[pllim]

Now some Travis tests failed

=================================== FAILURES ===================================

______________________ test_poisson_limit_nodependencies _______________________

    @pytest.mark.skipif('HAS_SCIPY or HAS_MPMATH')

    def test_poisson_limit_nodependencies():

        with pytest.raises(ImportError) as e:

>           a = funcs.poisson_conf_interval(20., 10., .95)

[mirca]

Although this problem seems insensitive for the choice of a prior, I think it should be stated somewhere that the code uses a positively defined uniform prior. (The authors also use exponential and Lorentzian priors, for comparison).

[hamogu]

I've added only a single line that's not covered by tests and that line is for compatibility with numpy 1.6 (where we don't run coveralls).
So, I request to ignore the coveralls failure here.

[hamogu]

@astrofrog : Can you restart the Travis tests? Seems that continuum.io was down (or things happened in ci-helpers ...)

[astrofrog]

See @cdeil's message on astropy-dev. I think this isn't fixed yet, but will try.

[hamogu]

I get the astropy-dev in "daily digest" so I did not see that yet. Thanks! Will wait until continuum's servers are up again.

[hamogu]

Any more comments anyone or can this be merged?

[astrofrog]

@crawfordsm @astronomeralex - any comments on this (as astropy.stats maintainers), or can we merge it?

[crawfordsm]

Finally got a chance to give this a final-review. Thanks for the contribution @hamogu !