Compatibility tweaks

Including this within a larger PHP project fails if that project already defines an __autoload function. Also, __autoload is set to be deprecated in PHP with spl_autoload the recommended replacement. 

Also, my instance of PHP did not like calling global on the $secp256k1_G object so I moved it into the (only) function where it is called.

phpecc had one additional commit for "speedup" that I've included.