Add missing javadoc required by Java 8

JDK 8 introduces `doclint` to validate the javadoc comments. This commit adds missing javadoc entries (required by `doclint`) and fixes a few violations. [Disabling doclint](http://blog.joda.org/2014/02/turning-off-doclint-in-jdk-8-javadoc.html) could be an alternative to this, but it was pretty easy to fix the violations.

Fixes #138

More info: http://openjdk.java.net/jeps/172