Add an abstract type HypothesisTest (#23)

This is currently defined in HypothesisTests. Defining the common
supertype in this package would allow other packages to define their own
hypothesis test types that are too specific and/or have too many
dependencies to upstream to HypothesisTests. This could include, for
example, the log-rank test for comparing Kaplan-Meier survival curves in
Survival and the likelihood ratio test in StatsModels.

src/StatsAPI.jl

@@ -32,6 +32,16 @@ function pairwise end
 # of the default definition.
 function pairwise! end
 
+"""
+    HypothesisTest
+
+Abstract supertype for all statistical hypothesis tests.
+Subtypes must implement [`pvalue`](@ref) at a minimum and may also
+implement functions such as [`confint`](@ref), [`nobs`](@ref), and
+[`dof`](@ref) as appropriate.
+"""
+abstract type HypothesisTest end
+
 """
     pvalue(test)