Issue #5: Generalize KeyStoreBasedSecretKeyFactoryBean.

Generify KeyStoreBasedSecretKeyFactoryBean to take parameter T->Key and
rename class to reflect general support for sensitive keys. Public keys
are not supported since they can't be stored directly in a KeyStore.
Merge pull request #8 from vt-middleware/generalize-ksbskfb

Issue #5: Generalize KeyStoreBasedSecretKeyFactoryBean.