[TIMOB-5496] Have an include_once function

An include_once function, such as what can be found in PHP would be extraordinarily useful on the mobile side to help prevent unnecessary duplicate inclusion in the same context. A good example is with the use of the date.js library. Including it twice in the same context results in recursion errors whenever the library's .toString() function is used (because it stores the prototype .toString in a new variable and replaces it with its own, the second include makes it replace the prototype backup with the custom function). Having the ability to use Ti.include_once() to prevent a file from being loaded twice would keep such an error from happening to begin with.