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.
Would you mind to elaborate on the reason that we need this function? Esp. I don't see it's used/tested in this PR.