Updated composer package info.

Add: PHP extension dependencies. Required by SmfRestClient.php
Change: Updated basic repo info.
Added gitignore file. Omit vendor dir.
Replaced echo with error_log().

The log message spat by the client will now be displayed in the configured php.ini log output. A common place for that under Linux would be the /var/log/syslog.
Moved SECRET_KEY constant into index.php file.

This constant should eventually be isolated into a separate config.php file. This way users won't have to modify the SmfRestServer or index files in order to specify
their API keys.
Minor syntax changes.