Unify the specification of distribution supports

[lindahua]

This PR introduces a new macro distr_support, which works for almost all univariate distributions that specify the support of the distribution. For example,

@distr_support Normal -Inf Inf
@distr_support Uniform d.a d.b
@distr_support Geometric 0 Inf

This macro can be used for various cases, no matter whether the support is bounded or not, or whether the support can be determined at the class-level or at the instance-level.

This macro will generate the following methods:

• insupport
• support, which returns a UnitRange instance for bounded discrete distribution or RealInterval for continuous distribution
• maximum
• minimum
• islowerbounded
• isupperbounded
• isbounded

So that one need not to rewrite these boilerplates when implementing a new distribution.

This macro has been successfully applied to all univariate distributions provided in the package.

[coveralls]

Coverage increased (+0.01%) when pulling f836759 on dh/support2 into 8184f0d on master.

[johnmyleswhite]

This is a big improvement. Thanks, Dahua!