Added unsigned integer types and change taosdemo insert and select queries for these types.

taos define | taos sql | c#
--- | --- | ---
UTINYINT | TINYINT UNSIGNED | `byte`
USMALLINT | SMALLINT UNSIGNED | `ushort`
UINT | INT UNSIGNED | `uint`
UBIGINT | BIGINT UNSIGNED | `ulong`
TINYINT | TINYINT | `sbyte`

Note: also change the tinyint value type to C# `sbyte`(the right signed one byte type) in this pr.

update README.md

support multithread create table and insertion.