[SPARK-4205][SQL] Timestamp and Date classes which work in the catalyst DSL.

These Timestamp and Date classes support the usual comparison operators, so they can be used in catalyst Expressions.  They come with implicit conversions to and from the underlying Java classes.  They also have a convenient initializer which accepts a String.

Additional implicit conversions fix an issue with the DSL where expressions like 0 < 'x are not recognized.

Sure? There's already an `append` method https://github.com/apache/spark/pull/3066/files#diff-b71f7eb2042698f801a345e8c7e90c5cR364 Otherwise the master branch couldn't have passed Jenkins build.