This PR is related with #517

• Add tx.MGet and tx.MSet. Use slices as the returned value.
• Add tx.Append.
• Add tx.GetRange. It will get the part by using a closed interval which composed with 2 int

tx.MGet and tx.MSet

• If calling them with 0 arg. Them will do nothing and return a nil.
• We must use an even number of args to call tx.MSet. If i is an even number, the ith arg and the i+1th arg will be a pair of k-v.

tx.Append

• If calling it with a empty appendage, it will do nothing and return a nil
• If use this method on a non-exist key, it will create a new value with the given appendage.

tx.GetRange

• Use start and end to be a closed interval and get the part of value by using it. (This is same with Redis)
• If start is greater than end, it will throw an error.
• If start is greater than the size of value, it will return a nil
• If end is greater than the size of value, it will return value[start:]
• Normally, it will return value[start:end+1]