Modified installer.php to accept flags for php path, proxy host

I ran in to a situation where I needed to pass flags to the "php" called by installer.php, and could not change the php.ini. I've modified installer.php to accept the --php="..." flag and moved the "dev" and "proxy" support to getopt as well.

I just noticed the guidelines link. I did not create this in a separate branch, I'm afraid. I will use a separate branch in future updates.