[ADD] Executes procedures and functions

[supr3m]

i needed execute a procedure mysql, that's it.

[avbdr]

thank you for your contribution. Patch looks very hovewer there are bunch of problems:

1. You have introduced new functionality but havent updated a readme examples on how to use them.
2. Patch is incompatible with old php because of [] vs array() usage.
3. Patch is incompatible with an ArrayBuilder/ObjectBuilder/JsonBuilder() modifiers
4. Im not sure if you need callFunction() as you still can use get("function(args)")
5. you are missing new tests for a new functionality

Can you please fix this issues and I will be glad to merge it.

[MPrtenjak]

i needed execute a procedure mysql, that's it.

I also needed to send NULL values so I have changed _validParams to support NULL values.

From this:

        foreach ( $params as $key => &$param ) {
                $param = "'". $this->mysqli()->real_escape_string( $param ) ."'";
        }

To this:

        foreach ( $params as $key => &$param ) {
            if ($param)
                $param = "'". $this->mysqli()->real_escape_string( $param ) ."'";
            else
                $param = 'NULL';
        }