Timestamp and Date classes which work in the catalyst DSL.

These Timestamp and Date classes support the usual comparison operators, so the can be used in catalyst Expressions.  The 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.

An independent change adds a method to Row for creating a row from a sequence of Strings and a schema.
It doesn't need to be a Row method.  I will move it outside ( and worry about the Boolean case.)

There is an issue lurking here, however.  A SchemaRDD should have Rows which match its schema.   But mismatches generate no errors until the last stage, e.g. when one collects the results of a query.  My thought was that it would be helpful to have a "safe" Row constructor which would check its data against a schema and raise an error if they do not match (after applying whatever implicit conversions are available.  (Yes, this is a separate issue and does not belong here.)