Added logDensity methods to AbstractReal/IntegerDistribution with naive default implementations and improved implementations for some current distributions.
JIRA: MATH-1039
Patch provided by Aleksei Dievskii

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/math/trunk@1533974 13f79535-47bb-0310-9956-ffa450edef68