Create a PropertiesConfiguration for `stormpath.*` properties

[mraible]

http://docs.spring.io/spring-boot/docs/current/reference/html/boot-features-external-config.html#boot-features-external-config-typesafe-configuration-properties

Doing this would allow others to inject our configuration into a Spring-managed class, rather than relying on individual Value annotations. Those would still work of course, but ConfigurationProperties is more inline with Spring Boot recommended practices.