Oracle: Added support for schemas (Issue #115)

[erezsh]

No description provided.

[sirupsen]

LGTM

Another problem he seemed to have was concat not supporting just 1 argument in Oracle, can you add this patch too to sql.py?

    def compile(self, c: Compiler):
        compiled_exprs = ", ".join(map(c.compile, self.exprs))
        expr = compiled_exprs
        if len(self.exprs) > 1:
            expr = f"concat({compiled_exprs})"
        md5 = c.database.md5_to_int(expr)
        return f"sum({md5})"