add support for SQL Server DateAdd()

[donshop11]

the following fails to parse:

select start_date where start_date > DATEADD(HH,-?, GETDATE())

Caused by: net.sf.jsqlparser.parser.ParseException: Encountered " <S_IDENTIFIER> "HH "" at line 1, column 46.

DATEADD() is describe here:
https://msdn.microsoft.com/en-us/library/ms186819.aspx

[donshop11]

it seems the problem is with the negated parameter, if you change it (remove the minus) it works:

select start_date where start_date > DATEADD(HH, ?, GETDATE())