Wrapping functions.php in an if-statement

This change wraps the functions defined in functions.php in an if
statement that checks to see if GUZZLE_FUNCTION_VERSION is defined. If
it is not defined, it is defined and set to the Guzzle version number.
This allows you to use a "global" version of Guzzle alongside a local version,
and still know which functions.php was autoloaded.

Closes #676