Optimization and bug fixes

- Fixed "out of memory" when passed empty string
- Function apn_free() and anp_payload_free() now declared as void and doesn't accept the second argument `error'
- Added function apn_set_mode()
- Added function apn_mode()
- Now function apn_init() accept new arguments: cert, private_key and private_key_pass
- Function apn_send() now doesn't accept argument `mode'
- Fixed hang when using the sandbox certificate in production mode
- Updated documentation