INTERVAL support, date pseudo literals TIMESTAMP and TIME literals.

[kchasapis]

The interval was not allowed to be a column data type. With this patch we add this support.

We add support for the pseudo literals CURRENT_TIME, CURRENT_TIMESTAMP, CURRENT_DATE.

Also, we add support for TIMESTAMP and TIME literals. The user can now explicitly set the type of the string value it follows with TIMESTAMP 'value' and TIME 'value.

DATE was already supported.

[kchasapis]

Thank you for your contribution! Before I go into detail reviewing your changes, can you please add some unit tests?

Thanks for your message. I have added unit tests. Please, let me know if there is anything else needed from my side to get started in the review.