Add support for AT TIME ZONE expressions

[tomershay]

Adding support for AT TIME ZONE expressions, used for example in PostgreSQL.
Sample usages:
SELECT creation_date AT TIME ZONE 'UTC';
SELECT DATE(date1 AT TIME ZONE 'UTC' AT TIME ZONE 'australia/sydney') AS another_date;

This PR doesn't cover all time zone related options that can be done on an expression, but is only a start.

Documentation and samples (some of the capabilities described aren't yet supported):
https://www.postgresql.org/docs/9.2/functions-datetime.html#FUNCTIONS-DATETIME-ZONECONVERT
https://www.enterprisedb.com/postgres-tutorials/postgres-time-zone-explained

This PR potentially fixes or makes progress with issues: #214, #1115

[coveralls]

Coverage increased (+0.1%) to 88.582% when pulling 1fd0c00 on tomershay:add_support_for_at_time_zone into 86b613c on JSQLParser:master.

[tomershay]

@wumpz, can you please share if there is a way to add further tests to increase coverage for the uncovered parts? Didn't see a way to do that as part of the existing testing suite. Thanks!

[manticore-projects]

Greetings @tomershay, I ran into the same issue a few days ago.

1. The Coveralls checks if each line of the Java Code has been executed/touched by your tests and the Details Report will show you which Java Code lines have NOT been touched

2. You will need to write tests, which trigger these Java Code Lines.

An over simplified example:

private int foo(int i) {
    if (i>0) {
         return 1;
    } else {
         return 0;
   }
}

@test 
private void fooTest() {
    assertEquals(1, foo(1));
}

The test above will execute the foo() method but will not run through all code lines and Coveralls would complain, that else return(0) is never touched.

You would need to add another test, that triggers that part.

[wumpz]

@tomershay Thx for adding this.