Support the current_timestamp() function

[akirk]

The function CURRENT_TIMESTAMP() is a synonym for NOW() which we already support.

cc @costasovo

[akirk]

Ok, I think I need some help here. I think the problem is that CURRENT_TIMESTAMP can also be a keyword in a field definition.

[JanJakes]

@akirk I think the issue is actually that in SQLite, the current_timestamp function can't be used with partentheses — SELECT CURRENT_TIMESTAMP works, SELECT CURRENT_TIMESTAMP() doesn't.

In a bigger picture:

1. CURRENT_TIMESTAMP is supported by SQLite, but only without parentheses, NOW() is not supported.
2. CURRENT_TIMESTAMP is SQL standard, NOW() is not.
3. In the current implementation, NOW() is emulated in PHP, although it would probably be more correct to just convert it to SQLite-native CURRENT_TIMESTAMP.
4. CURRENT_TIMESTAMP() can be only stripped off of the optional parentheses and consumed to the SQL query.

[JanJakes]

In short, I think the sqliteCreateFunction has no effect because CURRENT_TIMESTAMP is a SQLite-native function, so it probably cannot be overwritten. The quickest fix would be to remove the parentheses.

[JanJakes]

I took a stab at it in #151. It seems to work.