use new macros to declare the supports of distributions
export and test RealInterval
additional test of support-related invariance for discrete distributions
introduce ``@distr_support``
simplified the ``@distr_support`` macro
replace ``@continuous_distr_support`` with the new macro ``@distr_support``
use ``@distr_support`` for all univariate distributions
Merge pull request #312 from JuliaStats/dh/support2

Unify the specification of distribution supports