Fix javadoc issues reported by JDK 1.8.

On a system with oracle jdk 1.8 installed, the build without these
modifications fails due to a large number of problems reported for the
documentation.  This appears to be due to the newly introduced -Xdoclint
option to javadoc, which is enabled by default.
Merge pull request #13 from gagern/java8

Fix javadoc issues reported by JDK 1.8.