Refactored insert method and added singleQuote().

Refactored the insert method to add backticks around keys and use real_escape_string on strings or typecast int based on input. 

$items now accepts an associative array with the key being the column name and the value being another associative array of the type and value. Type assumes string but allows for int to be passed.

Added singleQuote method just to add single quotes around a string.