Add init_app function to (re-)initialize the app's configuration

init_app needs to be called if app.config is modified directly. I may change it in the future to be automatically called when the configuration dictionary is changed.
create_app can now handle multiple kinds of configuration

Before create_app could only take a filename to use as a configuration source. Now it accepts dictionary instances, strings naming an importable module, or naming a path to a configuration file in the "EVESRP_SETTINGS" environment variable in addition to the existing method of a path to a configuration file.