Add config/config.php to .gitignore
Add default config file. This contains the default values configured
for the application, while local app settings are stored in config.php
(which is .gitignore-d).
Add support for configuration files instead of editing index.php directly.

This patch loads configuration values from config/config.default.php and
config/config.php (if it exists), where the latter overwrites the default
settings from the former. Any app settings should be defined in config.php,
and config.default.php should not be touched.

This also changes references to constants in public/index.php to use the
values from the configuration files instead.
Merge pull request #2 from matslindh/rewrite-config-handling

Rewrite config handling to use local configuration files instead
Update configuration tutorial to the new configuration file format.
Merge pull request #3 from matslindh/update-configuration-tutorial

Update configuration tutorial to the new configuration file format.