Support for SQL Server and Oracle OFFSET ... FETCH ... clauses

The parser does not currently support new SQL Server 2012 and Oracle 12c OFFSET ... FETCH ... clauses.
This commit adds only a partial support of them, by allowing only integers for both clauses.

See http://technet.microsoft.com/en-gb/library/gg699618(v=sql.110).aspx for SQL Server and http://docs.oracle.com/database/121/SQLRF/statements_10002.htm#SQLRF55636 for Oracle.