Tajo-668: Add datetime function documentation

[eminency]

Like a kind of first draft...

Review in depth. It's much and rough.

[hyunsik]

Your patch includes some of Postgresql documentation. We already noticed that Tajo includes PostgreSQL. The license of PostgreSQL is compatible with ASF 2.0. In my view, the documentation looks good to me.

You said the documentation is rough. Could you let me know what parts do you want us to review intensively?

[eminency]

@hyunsik ,

Especially it means last part about conversion and formatting.

Like you said, it is almost quoted from PostgresSQL documentation. But I couldn't test for all features though I tested other basic examples. So I couldn't guarantee documentation contents because I don't have full knowledge of tajo datetime function implementation.

If there is no difference in actual datetime implementation between Tajo and Pgsql, I think it may be OK that you glance it.

[hyunsik]

For date/time conversion and formatting, we ported pgsql's implementation to Tajo. So, most of all features are compatible between Tajo and pgsql. So, it would be Ok.

@babokim could you look over this documentation? Because you contributed this feature, you may be the best reviewer on this issue.

[hyunsik]

+1 I'll commit it soon.

If we find something to be revise later, we can make additional jira and patch at that time.