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.
`ColumnType` classes are only used internally for in-memory columnar storage and generally don't need this kind of "convenient" methods.