Implements function `isCLI` to check if script is called from the CLI

*   Individual checks in every function were in place earlier.
Taint checking & wrapping `$_SERVER` in an ArrayAccess interface;

*   Abstract class `Tainted` provides basic abstraction for
*   implementing taint checking for various data types;
*   Class `TaintedString` uses the `__toString` magic function to
*   trigger user-level notices.
*   Class `HttpRequestArray` implements ArrayAccess and wraps
*   `$_SERVER`. It marks the `HTTP_*` indexes as contaminated.
Fixes indentation & spacing
Modified download class to include downloader